publications
all of my publications in reversed chronological order. generated by jekyll-scholar.
2022
The GALAH Survey: chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and Gaia eDR3
Sven Buder, Karin Lind, Melissa K. Ness, Diane K. Feuillet, Danny Horta, Stephanie Monty, Tobias Buck, Thomas Nordlander, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. Silva, Valentina DāOrazi, Ken C. Freeman, Michael R. Hayden, Janez Kos, Sarah L. Martell, Geraint F. Lewis, Jane Lin, Katharine J. Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Ioana CiucÄ, Jonathan Horner, Chiaki Kobayashi, Yuan-Sen Ting, Rosemary F. G. Wyse, and GALAH Collaboration WyseMNRAS, 2022Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus (GSE), appears to be an early ābuilding blockā given its virial mass \gt 10^10ā\mathrmM_ā at infall (z Ā 1-3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-Ī± abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via [Na/Fe] versus [Mg/Mn] in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including 30 \lt \sqrtJ_R / ā\mathrmkpc\,km\,s^-1 \lt 55, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for example mean \sqrtJ_R / ā\mathrmkpc\,km\,s^-1 =26_-14^+9. We find only (29\pm 1) \rm per cent of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
The GALAH Survey: improving our understanding of confirmed and candidate planetary systems with large stellar surveys
Jake T. Clark, Duncan J. Wright, Robert A. Wittenmyer, Jonathan Horner, Natalie R. Hinkel, Mathieu ClertĆ©, Brad D. Carter, Sven Buder, Michael R. Hayden, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Katharine J. Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Ulisse Munari, and Thomas NordlanderMNRAS, 2022Pioneering photometric, astrometric, and spectroscopic surveys is helping exoplanetary scientists better constrain the fundamental properties of stars within our galaxy and the planets these stars host. In this study, we use the third data release from the stellar spectroscopic GALAH Survey, coupled with astrometric data of eDR3 from the Gaia satellite, and other data from NASAās Exoplanet Archive, to refine our understanding of 279 confirmed and candidate exoplanet host stars and their exoplanets. This homogenously analysed data set comprises 105 confirmed exoplanets, along with 146 K2 candidates, 95 TESS Objects of Interest (TOIs), and 52 Community TOIs (CTOIs). Our analysis significantly shifts several previously (unknown) planet parameters while decreasing the uncertainties for others. Our radius estimates suggest that 35 planet candidates are more likely brown dwarfs or stellar companions due to their new radius values. We are able to refine the radii and masses of WASP-47 e, K2-106 b, and CoRoT-7 b to their most precise values yet to less than 2.3 per cent and 8.5 per cent, respectively. We also use stellar rotational values from GALAH to show that most planet candidates will have mass measurements that will be tough to obtain with current ground-based spectrographs. With GALAHās chemical abundances, we show through chemo-kinematics that there are five planet hosts that are associated with the galaxyās thick disc, including NGTS-4, K2-183, and K2-337. Finally, we show that there is no statistical difference between the chemical properties of hot Neptune and hot rocky exoplanet hosts, with the possibility that short-period rocky worlds might be the remnant cores of hotter, gaseous worlds.
The GALAH Survey: dependence of elemental abundances on age and metallicity for stars in the Galactic disc
Sanjib Sharma, Michael R. Hayden, Joss Bland-Hawthorn, Dennis Stello, Sven Buder, Joel C. Zinn, Lorenzo Spina, Thomas Kallinger, Martin Asplund, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Katharine J. Schlesinger, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Boquan Chen, Klemen Cotar, Prajwal R. Kafle, Shourya Khanna, Thor Tepper-Garcia, Purmortal Wang, and Rob A. WittenmyerMNRAS, 2022Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance-age-metallicity relations. Using a stochastic chemical enrichment scheme that takes the volume of supernovae remnants into account, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs, and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off (MSTO) stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of MSTO stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.
2021
The GALAH+ survey: a new library of observed stellar spectra improves radial velocities and hints at motions within M67
Tomaž Zwitter, Janez Kos, Sven Buder, Klemen Äotar, Martin Asplund, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Michael R. Hayden, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Katharine J. Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Kevin L. Beeson, Richard Grijs, Thomas Nordlander, Yuan-Sen Ting, Gregor Traven, Rok VogrinÄiÄ, Fred Watson, and Rob WittenmyerMNRAS, 2021GALAH+ is a magnitude-limited survey of high-resolution stellar spectra obtained by the HERMES spectrograph at the Australian Astronomical Observatory. Its third data release provides reduced spectra with new derivations of stellar parameters and abundances of 30 chemical elements for 584 015 dwarfs and giants, 88 per cent of them in the Gaia magnitude range 11 < G < 14. Here, we use these improved values of stellar parameters to build a library of observed spectra which is useful to study variations of individual spectral lines with stellar parameters. This and other improvements are used to derive radial velocities with uncertainties which are generally within 0.1 km s-1 or Ā 25 per cent smaller than in the previous release. Median differences in radial velocities measured here and by the Gaia DR2 or APOGEE DR16 surveys are smaller than 30 m s-1, a larger offset is present only for Gaia measurements of giant stars. We identify 4483 stars with intrinsically variable velocities and 225 stars for which the velocity stays constant over ā„3 visits spanning more than a year. The combination of radial velocities from GALAH+ with distances and sky plane motions from Gaia enables studies of dynamics within streams and clusters. For example, we estimate that the open cluster M67 has a total mass of Ā 3300 Mā and its outer parts seem to be expanding, though astrometry with a larger time-span than currently available from Gaia eDR3 is needed to judge if the latter result is real.
Measuring the Mass of the Large Magellanic Cloud with Stellar Streams Observed by Sāµ
Nora Shipp, Denis Erkal, Alex Drlica-Wagner, Ting S. Li, Andrew B. Pace, Sergey E. Koposov, Lara R. Cullinane, Gary S. Da Costa, Alexander P. Ji, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Zhen Wan, Daniel B. Zucker, Joss Bland-Hawthorn, Peter S. Ferguson, Sophia Lilleengen, and Sophia LilleengenApJ, 2021Stellar streams are excellent probes of the underlying gravitational potential in which they evolve. In this work, we fit dynamical models to five streams in the Southern Galactic hemisphere, combining observations from the Southern Stellar Stream Spectroscopic Survey (S 5), Gaia EDR3, and the Dark Energy Survey, to measure the mass of the Large Magellanic Cloud (LMC). With an ensemble of streams, we find a mass of the LMC ranging from Ā 14-19 Ć 1010 M ā, probed over a range of closest approach times and distances. With the most constraining stream (Orphan-Chenab), we measure an LMC mass of 18.8_-4.0^+3.5\times 10^10\,M_ā , probed at a closest approach time of 310 Myr and a closest approach distance of 25.4 kpc. This mass is compatible with previous measurements, showing that a consistent picture is emerging of the LMCās influence on structures in the Milky Way. Using this sample of streams, we find that the LMCās effect depends on the relative orientation of the stream and LMC at their point of closest approach. To better understand this, we present a simple model based on the impulse approximation and we show that the LMCās effect depends both on the magnitude of the velocity kick imparted to the stream and the direction of this kick.
Signature of a Massive Rotating Metal-poor Star Imprinted in the Phoenix Stellar Stream
Andrew R. Casey, Alexander P. Ji, Terese T. Hansen, Ting S. Li, Sergey E. Koposov, Gary S. Da Costa, Joss Bland-Hawthorn, Lara Cullinane, Denis Erkal, Geraint F. Lewis, Kyler Kuehn, Dougal Mackey, Sarah L. Martell, Andrew B. Pace, Jeffrey D. Simpson, and Daniel B. ZuckerApJ, 2021The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low-mass globular cluster. In this work we use Magellan/Magellan Inamori Kyocera Echelle (MIKE) high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ( Ļ([\mathrmFe\,\rm\smallII/\rmH])=0.04_-0.03^+0.11 ) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr II/Fe] spanning Ā 1 dex, while [Ba II/Fe] shows nearly no intrinsic spread ( Ļ([\mathrmBa\,\rm\smallII/\rmH])=0.03_-0.02^+0.10 ). This abundance signature is best interpreted as slow-neutron-capture element production from a massive fast-rotating metal-poor star (15-20 Mā, vini/vcrit = 0.4, [Fe/H] = -3.8). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron-capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant (A(Li) = 3.1 Ā± 0.1). At [Fe/H ] = -2.93; it is among the most metal-poor lithium-rich giants known. * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
Kinematics of Antlia 2 and Crater 2 from the Southern Stellar Stream Spectroscopic Survey (Sāµ)
Alexander P. Ji, Sergey E. Koposov, Ting S. Li, Denis Erkal, Andrew B. Pace, Joshua D. Simon, Vasily Belokurov, Lara R. Cullinane, Gary S. Da Costa, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Nora Shipp, Jeffrey D. Simpson, Daniel B. Zucker, Terese T. Hansen, Joss Bland-Hawthorn, and S5 CollaborationApJ, 2021We present new spectroscopic observations of the diffuse Milky Way satellite galaxies Antlia 2 and Crater 2, taken as part of the Southern Stellar Stream Spectroscopic Survey (S5). The new observations approximately double the number of confirmed member stars in each galaxy and more than double the spatial extent of spectroscopic observations in Antlia 2. A full kinematic analysis, including Gaia EDR3 proper motions, detects a clear velocity gradient in Antlia 2 and a tentative velocity gradient in Crater 2. The velocity gradient magnitudes and directions are consistent with particle stream simulations of tidal disruption. Furthermore, the orbit and kinematics of Antlia 2 require a model that includes the reflex motion of the Milky Way induced by the Large Magellanic Cloud. We also find that Antlia 2ās metallicity was previously overestimated, so it lies on the empirical luminosity-metallicity relation and is likely only now experiencing substantial stellar mass loss. Current dynamical models of Antlia 2 require it to have lost over 90% of its stars to tides, in tension with the low stellar mass loss implied by the updated metallicity. Overall, the new kinematic measurements support a tidal disruption scenario for the origin of these large and extended dwarf spheroidal galaxies.
Sāµ: The Orbital and Chemical Properties of One Dozen Stellar Streams
Ting S. Li, Alexander P. Ji, Andrew B. Pace, Denis Erkal, Sergey E. Koposov, Nora Shipp, Gary S. Da Costa, Lara R. Cullinane, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Daniel B. Zucker, Peter S. Ferguson, Sarah L. Martell, Joss Bland-Hawthorn, Eduardo Balbinot, Kiyan Tavangar, Alex Drlica-Wagner, Gayandhi M. De Silva, Joshua D. Simon, and S5 Collaboration2021We report the kinematic, orbital, and chemical properties of 12 stellar streams with no evident progenitors, using line-of-sight velocities and metallicities from the Southern Stellar Stream Spectroscopic Survey (S^5), proper motions from Gaia EDR3, and distances derived from distance tracers or the literature. This data set provides the largest homogeneously analyzed set of streams with full 6D kinematics and metallicities. All streams have heliocentric distances between ā¼10-50 kpc. The velocity and metallicity dispersions show that half of the stream progenitors were disrupted dwarf galaxies (DGs), while the other half originated from disrupted globular clusters (GCs), hereafter referred to as DG and GC streams. Based on the mean metallicities of the streams and the mass-metallicity relation, the luminosities of the progenitors of the DG streams range between Carina and Ursa Major I (-9.5ā²M_Vā²-5.5). Four of the six GC streams have mean metallicities of [Fe/H]< -2, more metal-poor than typical Milky Way (MW) GCs at similar distances. Interestingly, the 300S and Jet GC streams are the only streams on retrograde orbits in our dozen stream sample. Finally, we compare the orbital properties of the streams with known DGs and GCs in the MW, finding several possible associations. Some streams appear to have been accreted with the recently discovered Gaia-Enceladus-Sausage system, and others suggest that GCs were formed in and accreted together with the progenitors of DG streams whose stellar masses are similar to Draco to Carina (\sim10^5-10^6M_ā).
The GALAH survey: effective temperature calibration from the InfraRed Flux Method in the Gaia system
Luca Casagrande, Jane Lin, Adam D. Rains, Fan Liu, Sven Buder, Jonathan Horner, Martin Asplund, Geraint F. Lewis, Sarah L. Martell, Thomas Nordlander, Dennis Stello, Yuan-Sen Ting, Robert A. Wittenmyer, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Michael R. Hayden, Janez Kos, Karin Lind, Katharine J. Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Daniel B. Zucker, and Tomaž ZwitterMNRAS, 2021In order to accurately determine stellar properties, knowledge of the effective temperature of stars is vital. We implement Gaia and 2MASS photometry in the InfraRed Flux Method and apply it to over 360 000 stars across different evolutionary stages in the GALAH DR3 survey. We derive colour-effective temperature relations that take into account the effect of metallicity and surface gravity over the range 4000ā\rm Kā²T_\rm effā²8000ā\rm K, from very metal-poor stars to supersolar metallicities. The internal uncertainty of these calibrations is of order 40-80 K depending on the colour combination used. Comparison against solar-twins, Gaia benchmark stars, and the latest interferometric measurements validates the precision and accuracy of these calibrations from F to early M spectral types. We assess the impact of various sources of uncertainties, including the assumed extinction law, and provide guidelines to use our relations. Robust solar colours are also derived.
The GALAH survey: accreted stars also inhabit the Spite plateau
Jeffrey D. Simpson, Sarah L. Martell, Sven Buder, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. Silva, Valentina DāOrazi, Ken C. Freeman, Michael Hayden, Janez Kos, Geraint F. Lewis, Karin Lind, Katharine J. Schlesinger, Sanjib Sharma, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Martin Asplund, Gary Costa, Klemen Äotar, Thor Tepper-GarcĆa, Jonathan Horner, Thomas Nordlander, Yuan-Sen Ting, Rosemary F. G. Wyse, and GALAH CollaborationMNRAS, 2021The European Space Agency (ESA) Gaia mission has enabled the remarkable discovery that a large fraction of the stars near the solar neighbourhood are debris from a single in-falling system, the so-called Gaia-Sausage-Enceladus (GSE). This discovery provides astronomers for the first time with a large cohort of easily observable, unevolved stars that formed in a single extragalactic environment. Here we use these stars to investigate the āSpite plateauā - the near-constant lithium abundance observed in unevolved metal-poor stars across a wide range of metallicities (-3 < [Fe/H] < -1). Our aim is to test whether individual galaxies could have different Spite plateaus - e.g. the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We identified 93 GSE dwarf stars observed and analysed by the GALactic Archaeology with HERMES (GALAH) survey as part of its Data Release 3 (DR3). Orbital actions were used to select samples of GSE stars, and comparison samples of halo and disc stars. We find that the GSE stars show the same lithium abundance as other likely accreted stars and in situ Milky Way stars. Formation environment leaves no imprint on lithium abundances. This result fits within the growing consensus that the Spite plateau, and more generally the ācosmological lithium problemā - the observed discrepancy between the amount of lithium in warm, metal-poor dwarf stars in our Galaxy, and the amount of lithium predicted to have been produced by big bang nucleosynthesis - is the result of lithium depletion processes within stars.
The GALAH survey: Chemical homogeneity of the Orion complex
Janez Kos, Joss Bland-Hawthorn, Sven Buder, Thomas Nordlander, Lorenzo Spina, Kevin L. Beeson, Karin Lind, Martin Asplund, Ken Freeman, Michael R. Hayden, Geraint F. Lewis, Sarah L. Martell, Sanjib Sharma, Gayandhi De Silva, Jefferey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Klemen Äotar, Jonti Horner, Yuan-Sen Ting, and Gregor TravenMNRAS, 2021Due to its proximity, the Orion star forming region is often used as a proxy to study processes related to star formation and to observe young stars in the environment they were born in. With the release of Gaia DR2, the distance measurements to the Orion complex are now good enough that the 3D structure of the complex can be explored. Here we test the hypothesis that, due to non-trivial structure and dynamics, and age spread in the Orion complex, the chemical enrichment of youngest stars by early core-collapse supernovae can be observed. We obtained spectra of 794 stars of the Orion complex with the HERMES spectrograph at the Anglo Australian telescope as a part of the GALAH and GALAH-related surveys. We use the spectra of Ā 300 stars to derive precise atmospheric parameters and chemical abundances of 25 elements for 15 stellar clusters in the Orion complex. We demonstrate that the Orion complex is chemically homogeneous and that there was no self-pollution of young clusters by core-collapse supernovae from older clusters; with a precision of 0.02 dex in relative alpha-elements abundance and 0.06 dex in oxygen abundance we would have been able to detect pollution from a single supernova, given a fortunate location of the SN and favourable conditions for ISM mixing. We estimate that the supernova rate in the Orion complex was very low, possibly producing no supernova by the time the youngest stars of the observed population formed (from around 21 to 8 Myr ago).
Fundamental relations for the velocity dispersion of stars in the Milky Way
Sanjib Sharma, Michael R. Hayden, Joss Bland-Hawthorn, Dennis Stello, Sven Buder, Joel C. Zinn, Thomas Kallinger, Martin Asplund, Gayandhi M. De Silva, Valentina DāOrazi, Ken Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah Martell, Jeffrey D. Simpson, Rob A. Wittenmyer, Daniel B. Zucker, Tomaz Zwitter, Boquan Chen, Klemen Cotar, James Esdaile, Marc Hon, Jonathan Horner, Daniel Huber, Prajwal R. Kafle, Shourya Khanna, Yuan-Sen Ting, David M. Nataf, Thomas Nordlander, Mohd Hafiz Mohd Saadon, Thor Tepper-Garcia, C. G. Tinney, Gregor Traven, Fred Watson, Duncan Wright, and Rosemary F. G. WyseMNRAS, 2021We explore the fundamental relations governing the radial and vertical velocity dispersions of stars in the Milky Way, from combined studies of complementary surveys including GALAH, LAMOST, APOGEE, the NASA Kepler and K2 missions, and Gaia DR2. We find that different stellar samples, even though they target different tracer populations and employ a variety of age estimation techniques, follow the same set of fundamental relations. We provide the clearest evidence to date that, in addition to the well-known dependence on stellar age, the velocity dispersions of stars depend on orbital angular momentum Lz, metallicity, and height above the plane |z|, and are well described by a multiplicatively separable functional form. The dispersions have a power-law dependence on age with exponents of 0.441 Ā± 0.007 and 0.251 Ā± 0.006 for Ļz and ĻR, respectively, and the power law is valid even for the oldest stars. For the solar neighbourhood stars, the apparent break in the power law for older stars, as seen in previous studies, is due to the anticorrelation of Lz with age. The dispersions decrease with increasing Lz until we reach the Sunās orbital angular momentum, after which Ļz increases (implying flaring in the outer disc) while ĻR flattens. For a given age, the dispersions increase with decreasing metallicity, suggesting that the dispersions increase with birth radius. The dispersions also increase linearly with |z|. The same set of relations that work in the solar neighbourhood also work for stars between 3 < R/kpc < 20. Finally, the high-[Ī±/Fe] stars follow the same relations as the low-[Ī±/Fe] stars.
The GALAH+ survey: Third data release
Sven Buder, Sanjib Sharma, Janez Kos, Anish M. Amarsi, Thomas Nordlander, Karin Lind, Sarah L. Martell, Martin Asplund, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. Silva, Valentina DāOrazi, Ken C. Freeman, Michael R. Hayden, Geraint F. Lewis, Jane Lin, Katharine J. Schlesinger, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Kevin L. Beeson, Tobias Buck, Luca Casagrande, Jake T. Clark, Klemen Äotar, Gary S. Costa, Richard Grijs, Diane Feuillet, Jonathan Horner, Prajwal R. Kafle, Shourya Khanna, Chiaki Kobayashi, Fan Liu, Benjamin T. Montet, Govind Nandakumar, David M. Nataf, Melissa K. Ness, Lorenzo Spina, Thor Tepper-GarcĆa, Yuan-Sen Ting, Gregor Traven, Rok VogrinÄiÄ, Robert A. Wittenmyer, Rosemary F. G. Wyse, MaruÅ”a Žerjal, and GALAH CollaborationMNRAS, 2021The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2 per cent of stars are within <2 kpc), observed with the HERMES spectrograph at the Anglo-Australian Telescope. This release (hereafter GALAH+ DR3) includes all observations from GALAH Phase 1 (bright, main, and faint survey, 70 per cent), K2-HERMES (17 per cent), TESS-HERMES (5 per cent), and a subset of ancillary observations (8 per cent) including the bulge and >75 stellar clusters. We derive stellar parameters Teff, log g, [Fe/H], vmic, vbroad, and vrad using our modified version of the spectrum synthesis code Spectroscopy Made Easy (SME) and 1D MARCS model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from Gaia DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65 per cent dwarfs, 34 per cent giants, and 1 per cent other/unclassified stars. Based on unflagged chemical composition and age, we find 62 per cent young low-Ī±, 9 per cent young high-Ī±, 27 per cent old high-Ī±, and 2 per cent stars with [Fe/H] ā¤ -1. Based on kinematics, 4 per cent are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after Gaia eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.
The GALAH survey and symbiotic stars - I. Discovery and follow-up of 33 candidate accreting-only systems
U. Munari, G. Traven, N. Masetti, P. Valisa, G. -L. Righetti, F. -J. Hambsch, A. Frigo, K. Äotar, G. M. De Silva, K. C. Freeman, G. F. Lewis, S. L. Martell, S. Sharma, J. D. Simpson, Y. -S. Ting, R. A. Wittenmyer, and D. B. ZuckerMNRAS, 2021We have identified a first group of 33 new candidates for symbiotic stars (SySt) of the accreting-only variety among the 600 255 stars so far observed by the GALactic Archaeology with HERMES (GALAH) high-resolution spectroscopic survey of the Southern hemisphere, more than doubling the number of those previously known. GALAH aims to high latitudes and this offers the possibility to sound the Galaxy for new SySt away from the usual plane and bulge hunting regions. In this paper, we focus on SySt of the M spectral type, showing an HĪ± emission with a peak in excess of 0.5 above the adjacent continuum level, and not affected by coherent radial pulsations. These constraints will be relaxed in future studies. The 33 new candidate SySt were subjected to an array of follow-up confirmatory observations [X-ray/ultraviolet (UV) observations with the Swift satellite, search for optical flickering, presence of a near-UV upturn in ground-based photometric and spectroscopic data, radial velocity changes suggestive of orbital motion, and variability of the emission-line profiles]. According to Gaia Early Data Release 3 (EDR3) parallaxes, the candidate new SySt are located at the tip of the giant branch, sharing the same distribution in M(Ks) of the well-established SySt. The accretion luminosities of the candidate new SySt are in the range 1-10 Lā, corresponding to mass accretion rates of 0.1-1 Ć 10-9 Mā yr-1 for white dwarfs of 1 Mā. The M giant of one of them presents a large lithium overabundance.
The GALAH survey: a census of lithium-rich giant stars
Sarah L. Martell, Jeffrey D. Simpson, Adithya G. Balasubramaniam, Sven Buder, Sanjib Sharma, Marc Hon, Dennis Stello, Yuan-Sen Ting, Martin Asplund, Joss Bland-Hawthorn, Gayandhi M. De Silva, Ken C. Freeman, Michael Hayden, Janez Kos, Geraint F. Lewis, Karin Lind, Daniel B. Zucker, Tomaž Zwitter, Simon W. Campbell, Klemen Äotar, Jonathan Horner, Benjamin Montet, and Rob WittenmyerMNRAS, 2021We investigate the properties of 1262 red giant stars with high photospheric abundances of lithium observed by the GALAH and K2-HERMES surveys, and discuss them in the context of proposed mechanisms for lithium enrichment and redepletion in giant stars. We confirm that Li-rich giants are rare, making up only 1.2 per cent of our giant star sample. We use stellar parameters from the third public data release from the GALAH survey and a Bayesian isochrone analysis to divide the sample into first-ascent red giant branch (RGB) and red clump (RC) stars, and confirm these classifications using asteroseismic data from K2. We find that RC stars are 2.5 times as likely to be lithium-rich as RGB stars, in agreement with other recent work. The probability for a star to be lithium-rich is affected by a number of factors, though the causality in those correlations is not entirely clear. We show for the first time that primary and secondary RC stars have distinctly different lithium enrichment patterns. The data set discussed here is large and heterogeneous in terms of evolutionary phase, metallicity, rotation rate, and mass. We expect that if the various mechanisms that have been proposed for lithium enrichment in evolved stars are in fact active, they should all contribute to this sample of lithium-rich giants at some level.
The GALAH Survey: using galactic archaeology to refine our knowledge of TESS target stars
Jake T. Clark, Mathieu ClertĆ©, Natalie R. Hinkel, Cayman T. Unterborn, Robert A. Wittenmyer, Jonathan Horner, Duncan J. Wright, Brad Carter, Timothy D. Morton, Lorenzo Spina, Martin Asplund, Sven Buder, Joss Bland-Hawthorn, Andy Casey, Gayandhi De Silva, Valentina DāOrazi, Ly Duong, Michael Hayden, Ken Freeman, Janez Kos, Geraint Lewis, Jane Lin, Karin Lind, Sarah Martell, Sanjib Sharma, Jeffrey Simpson, Dan Zucker, Tomaz Zwitter, Christopher G. Tinney, Yuan-Sen Ting (äøęŗę£®), Thomas Nordlander, and Anish M. AmarsiMNRAS, 2021An unprecedented number of exoplanets are being discovered by the Transiting Exoplanet Survey Satellite (TESS). Determining the orbital parameters of these exoplanets, and especially their mass and radius, will depend heavily upon the measured physical characteristics of their host stars. We have cross-matched spectroscopic, photometric, and astrometric data from GALAH Data Release 2, the TESS Input Catalog and Gaia Data Release 2, to create a curated, self-consistent catalogue of physical and chemical properties for 47 285 stars. Using these data, we have derived isochrone masses and radii that are precise to within 5 per cent. We have revised the parameters of three confirmed, and twelve candidate, TESS planetary systems. These results cast doubt on whether CTOI-20125677 is indeed a planetary system, since the revised planetary radii are now comparable to stellar sizes. Our GALAH-TESS catalogue contains abundances for up to 23 elements. We have specifically analysed the molar ratios for C/O, Mg/Si, Fe/Si, and Fe/Mg, to assist in determining the composition and structure of planets with Rp < 4Rā. From these ratios, 36 per cent fall within 2Ļsigma of the Sun/Earth values, suggesting that these stars may host rocky exoplanets with geological compositions similar to planets found within our own Solar system.
Sāµ: The Destruction of a Bright Dwarf Galaxy as Revealed by the Chemistry of the Indus Stellar Stream
Terese T. Hansen, Alexander P. Ji, Gary S. Da Costa, Ting S. Li, Andrew R. Casey, Andrew B. Pace, Lara R. Cullinane, Denis Erkal, Sergey E. Koposov, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Nora Shipp, Daniel B. Zucker, Joss Bland-Hawthorn, and S5 CollaborationApJ, 2021The recently discovered Indus stellar stream exhibits a diverse chemical signature compared to what is found for most other streams due to the abundances of two outlier stars, Indus_0 and Indus_13. Indus_13 exhibits an extreme enhancement in rapid neutron-capture (r-)process elements with [Eu/Fe] = + 1.81. It thus provides direct evidence of the accreted nature of r-process-enhanced stars. In this paper we present a detailed chemical analysis of the neutron-capture elements in Indus_13, revealing the star to be slightly actinide poor. The other outlier, Indus_0, displays a globular cluster-like signature with high N, Na, and Al abundances, while the rest of the Indus stars show abundances compatible with a dwarf galaxy origin. Hence, Indus_0 provides the first chemical evidence of a fully disrupted dwarf containing a globular cluster. We use the chemical signature of the Indus stars to discuss the nature of the stream progenitor which was likely a chemically evolved system, with a mass somewhere in the range from Ursa Minor to Fornax.
The GALAH survey: tracing the Galactic disc with open clusters
L. Spina, Y. -S. Ting, G. M. De Silva, N. Frankel, S. Sharma, T. Cantat-Gaudin, M. Joyce, D. Stello, A. I. Karakas, M. B. Asplund, T. Nordlander, L. Casagrande, V. DāOrazi, A. R. Casey, P. Cottrell, T. Tepper-GarcĆa, M. Baratella, J. Kos, K. Äotar, J. Bland-Hawthorn, S. Buder, K. C. Freeman, M. R. Hayden, G. F. Lewis, J. Lin, K. Lind, S. L. Martell, K. J. Schlesinger, J. D. Simpson, D. B. Zucker, and T. ZwitterMNRAS, 2021Open clusters are unique tracers of the history of our own Galaxyās disc. According to our membership analysis based on Gaia astrometry, out of the 226 potential clusters falling in the footprint of the GALactic Archaeology with HERMES (GALAH) survey or the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we find that 205 have secure members that were observed by at least one of the surveys. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disc of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is -0.076 Ā± 0.009 dex kpc-1, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H]-guiding radius (rguid) plane is -0.073 Ā± 0.008 dex kpc-1. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disc differently than field stars. In particular, at the given radius, open clusters show an age-metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]-rguid-age space, which are important to understand production rates of different elements as a function of space and time.
The GALAH Survey: No Chemical Evidence of an Extragalactic Origin for the Nyx Stream
Daniel B. Zucker, Jeffrey D. Simpson, Sarah L. Martell, Geraint F. Lewis, Andrew R. Casey, Yuan-Sen Ting, Jonathan Horner, Thomas Nordlander, Rosemary F. G. Wyse, Tomaž Zwitter, Joss Bland-Hawthorn, Sven Buder, Martin Asplund, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Michael R. Hayden, Janez Kos, Jane Lin, Karin Lind, Katharine J. Schlesinger, Sanjib Sharma, and Dennis StelloApJL, 2021The results from the ESA Gaia astrometric mission and deep photometric surveys have revolutionized our knowledge of the Milky Way. There are many ongoing efforts to search these data for stellar substructure to find evidence of individual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk. Using a combination of elemental abundances and stellar parameters from the GALAH and Apache Point Observatory Galactic Evolution Experiment (APOGEE) surveys, we find that the abundances of the highest likelihood Nyx members are entirely consistent with membership of the thick disk, and inconsistent with a dwarf galaxy origin. We conclude that the postulated Nyx stream is most probably a high-velocity component of the Milky Wayās thick disk. With the growing availability of large data sets including kinematics, stellar parameters, and detailed abundances, the probability of detecting chance associations increases, and hence new searches for substructure require confirmation across as many data dimensions as possible.
Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream
Ting S. Li, Sergey E. Koposov, Denis Erkal, Alexander P. Ji, Nora Shipp, Andrew B. Pace, Tariq Hilmi, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Zhen Wan, Daniel B. Zucker, Joss Bland-Hawthorn, Lara R. Cullinane, Gary S. Da Costa, Alex Drlica-Wagner, Kohei Hattori, Sarah L. Martell, Sanjib Sharma, and S5 CollaborationApJ, 2021We present the first spectroscopic measurements of the ATLAS and Aliqa Uma streams from the Southern Stellar Stream Spectroscopic Survey (S5), in combination with the photometric data from the Dark Energy Survey and astrometric data from Gaia. From the coherence of spectroscopic members in radial velocity and proper motion, we find that these two systems are extremely likely to be one stream with discontinuity in morphology and density on the sky (the "kink" feature). We refer to this entire stream as the ATLAS-Aliqa Uma stream, or the AAU stream. We perform a comprehensive exploration of the effect of baryonic substructures and find that only an encounter with the Sagittarius dwarf ā¼0.5 Gyr ago can create a feature similar to the observed "kink." In addition, we also identify two gaps in the ATLAS component associated with the broadening in the stream width (the "broadening" feature). These gaps have likely been created by small mass perturbers, such as dark matter halos, as the AAU stream is the most distant cold stream known with severe variations in both the stream surface density and the stream track on the sky. With the stream track, stream distance, and kinematic information, we determine the orbit of the AAU stream and find that it has been affected by the Large Magellanic Cloud, resulting in a misalignment between the proper motion and stream track. Together with the Orphan-Chenab Stream, AAU is the second stream pair that has been found to be a single stream separated into two segments by external perturbation.
2020
The GALAH Survey: Chemical Clocks
Michael R. Hayden, Sanjib Sharma, Joss Bland-Hawthorn, Lorenzo Spina, Sven Buder, Martin Asplund, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Katharine J. Schlesinger, Jeffrey D. Simpson, Daniel B. Zucker, Tomaz Zwitter, Boquan Chen, Klemen Cotar, Diane Feuillet, Jonti Horner, Meridith Joyce, Thomas Nordlander, Dennis Stello, Thorsten Tepper-Garcia, Yuan-sen Ting, Purmortal Wang, and Rob Wittenmyer2020Previous studies have found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for a sample of 250,000 stars taken from GALAH DR3 using only their overall metallicity and chemical abundances. Stellar ages are estimated via the machine learning algorithm XGBoost, using main sequence turnoff stars with precise ages as our input training set. We find that the stellar ages for the bulk of the GALAH DR3 sample are accurate to 1-2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disk, including the age-velocity dispersion relationship of the solar neighborhood and the larger global velocity dispersion relations of the disk found using Gaia and GALAH. The fact that chemical abundances alone can be used to determine a reliable age for a star have profound implications for the future study of the Galaxy as well as upcoming spectroscopic surveys. These results show that the chemical abundance variation at a given birth radius is quite small, and imply that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need of spectroscopic surveys to deliver precision abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages for stars directly from their chemical abundances. Applying the methods outlined in this paper opens a new door into studies of the kinematic structure and evolution of the disk, as ages may potentially be estimated for a large fraction of stars in existing spectroscopic surveys. This would yield a sample of millions of stars with reliable age determinations, and allow precise constraints to be put on various kinematic processes in the disk, such as the efficiency and timescales of radial migration.
The GALAH survey: Milky Way disc metallicity and alpha-abundance trends in combined APOGEE-GALAH catalogues
Govind Nandakumar, Michael R. Hayden, Sanjib Sharma, Sven Buder, Martin Asplund, Joss Bland-Hawthorn, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Sarah L. Martell, Katharine J. Schlesinger, Jane Lin, Jeffrey D. Simpson, Daniel B. Zucker, Tomaz Zwitter, Thomas Nordlander, Luca Casagrande, Karin Lind, Klemen Cotar, Dennis Stello, Robert A. Wittenmyer, and Thor Tepper-Garcia2020GALAH and APOGEE are two high resolution multi object spectroscopic surveys that provide fundamental stellar parameters and multiple elemental abundance estimates for > 400,000 stars in the Milky Way. They are complimentary in both sky coverage and wavelength regime. Thus combining the two surveys will provide us a large sample to investigate the disc metallicity and alpha abundance trends. We use the Cannon data-driven approach selecting training sets from among \sim20,000 stars in common for the two surveys to predict the GALAH scaled stellar parameters from APOGEE spectra as well as APOGEE scaled stellar parameters from GALAH spectra. We provide two combined catalogues with GALAH scaled and APOGEE scaled stellar parameters each having \sim500,000 stars after quality cuts. With \sim470,000 stars that are common in both these catalogues, we compare the GALAH scaled and APOGEE scaled metallicity distribution functions (MDF), radial and vertical metallicity gradients as well as the variation of [Ī±/Fe] vs [Fe/H] trends along and away from the Galactic mid plane. We find mean metallicities of APOGEE scaled sample to be higher compared to that for the GALAH scaled sample. We find similar [Ī±/Fe] vs [Fe/H] trends using both samples consistent with previous observational as well as simulation based studies. Radial and vertical metallicity gradients derived using the two survey scaled samples are consistent except in the inner and outer Galactocentric radius bins. Our gradient estimates in the solar neighborhood are also consistent with previous studies and are backed by larger sample size compared to previous works.
The Southern Stellar Stream Spectroscopic Survey (Sāµ): Chemical Abundances of Seven Stellar Streams
Alexander P. Ji, Ting S. Li, Terese T. Hansen, Andrew R. Casey, Sergey E. Koposov, Andrew B. Pace, Dougal Mackey, Geraint F. Lewis, Jeffrey D. Simpson, Joss Bland-Hawthorn, Lara R. Cullinane, Gary. S. Da Costa, Kohei Hattori, Sarah L. Martell, Kyler Kuehn, Denis Erkal, Nora Shipp, Zhen Wan, and Daniel B. ZuckerAJ, 2020We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic Survey (S5): ATLAS, Aliqa Uma, Chenab, Elqui, Indus, Jhelum, and Phoenix. Abundances of 30 elements have been derived from over 10,000 individual line measurements or upper limits using photometric stellar parameters and a standard LTE analysis. This is currently the most extensive set of element abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and Phoenix) are disrupted metal-poor globular clusters, although only weak evidence is seen for the light-element anticorrelations commonly observed in globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are disrupted dwarf galaxies, and their stars display abundance signatures that suggest progenitors with stellar masses ranging from 106 to 107 Mā. Extensive description is provided for the analysis methods, including the derivation of a new method for including the effect of stellar parameter correlations on each starās abundance and uncertainty. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
The GALAH Survey: non-LTE departure coefficients for large spectroscopic surveys
A. M. Amarsi, K. Lind, Y. Osorio, T. Nordlander, M. Bergemann, H. Reggiani, E. X. Wang, S. Buder, M. Asplund, P. S. Barklem, A. Wehrhahn, Ć. SkĆŗladĆ³ttir, C. Kobayashi, A. I. Karakas, X. D. Gao, J. Bland-Hawthorn, G. M. Silva, J. Kos, G. F. Lewis, S. L. Martell, S. Sharma, J. D. Simpson, D. B. Zucker, K. Äotar, J. Horner, and GALAH CollaborationA&A, 2020Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D MARCS model atmospheres that spans 3000 ā¤ Teff/K ā¤ 8000, - 0.5 ā¤log g/cm s-2 ā¤ 5.5, and - 5 ā¤ [Fe/H] ā¤ 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between - 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the [A/Fe] versus [Fe/H] plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys.
Grids of departure coefficients can be found online (<xref ref-type="bibr" rid="R3">http://Amarsi 2020</xref>http://) or by contacting the lead author directly.The GALAH survey: a new constraint on cosmological lithium and Galactic lithium evolution from warm dwarf stars
Xudong Gao, Karin Lind, Anish M. Amarsi, Sven Buder, Joss Bland-Hawthorn, Simon W. Campbell, Martin Asplund, Andrew R. Casey, Gayandhi M. Silva, Ken C. Freeman, Michael R. Hayden, Geraint F. Lewis, Sarah L. Martell, Jeffrey D. Simpson, Sanjib Sharma, Daniel B. Zucker, Tomaž Zwitter, Jonathan Horner, Ulisse Munari, Thomas Nordlander, Dennis Stello, Yuan-Sen Ting, Gregor Traven, Robert A. Wittenmyer, and GALAH CollaborationMNRAS, 2020Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution analysis of Li abundances A(Li) to date, with results for over 100ā000 GALAH (Galactic Archeology with HERMES) field stars spanning effective temperatures 5900ā\mathrmK ā²T_\mathrmeffā²7000ā\mathrmK and metallicities -3 ā² [Fe/H] ā² +0.5. We separated these stars into two groups, on the warm and cool sides of the so-called Li dip, a localized region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)-[Fe/H] plane, but with a roughly constant offset in A(Li) of 0.4ā\mathrmdex , the warm group having higher Li abundances. At \rm [Fe/H]ā³-0.5 , a significant increase in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around 0.4ā\mathrmdex relative to the primordial value predicted from big bang nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = -1.0 and -0.5 form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.
The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way
Anke Arentsen, Else Starkenburg, Nicolas F. Martin, David S. Aguado, Daniel B. Zucker, Carlos Allende Prieto, Vanessa Hill, Kim A. Venn, Raymond G. Carlberg, Jonay I. GonzĆ”lez HernĆ”ndez, Lyudmila I. Mashonkina, Julio F. Navarro, RubĆ©n SĆ”nchez-Janssen, Mathias Schultheis, Guillaume F. Thomas, Kris Youakim, Geraint F. Lewis, Jeffrey D. Simpson, Zhen Wan, Roger E. Cohen, Doug Geisler, and Julia E. OāConnellMNRAS, 2020Metal-poor stars are important tools for tracing the early history of the Milky Way, and for learning about the first generations of stars. Simulations suggest that the oldest metal-poor stars are to be found in the inner Galaxy. Typical bulge surveys, however, lack low metallicity ( \rm [Fe/H] \lt -1.0 ) stars because the inner Galaxy is predominantly metal-rich. The aim of the Pristine Inner Galaxy Survey (PIGS) is to study the metal-poor and very metal-poor (VMP, \rm [Fe/H] \lt -2.0 ) stars in this region. In PIGS, metal-poor targets for spectroscopic follow-up are selected from metallicity-sensitive CaHK photometry from the CFHT. This work presents the ā¼250 deg2 photometric survey as well as intermediate-resolution spectroscopic follow-up observations for ā¼8000 stars using AAOmega on the AAT. The spectra are analysed using two independent tools: ULySS with an empirical spectral library, and FERRE with a library of synthetic spectra. The comparison between the two methods enables a robust determination of the stellar parameters and their uncertainties. We present a sample of 1300 VMP stars - the largest sample of VMP stars in the inner Galaxy to date. Additionally, our spectroscopic data set includes ā¼1700 horizontal branch stars, which are useful metal-poor standard candles. We furthermore show that PIGS photometry selects VMP stars with unprecedented efficiency: 86 per cent/80 per cent (lower/higher extinction) of the best candidates satisfy \rm [Fe/H] \lt -2.0 , as do 80 per cent/63 per cent of a larger, less strictly selected sample. We discuss future applications of this unique data set that will further our understanding of the chemical and dynamical evolution of the innermost regions of our Galaxy.
The tidal remnant of an unusually metal-poor globular cluster
Zhen Wan, Geraint F. Lewis, Ting S. Li, Jeffrey D. Simpson, Sarah L. Martell, Daniel B. Zucker, Jeremy R. Mould, Denis Erkal, Andrew B. Pace, Dougal Mackey, Alexander P. Ji, Sergey E. Koposov, Kyler Kuehn, Nora Shipp, Eduardo Balbinot, Joss Bland-Hawthorn, Andrew R. Casey, Gary S. Da Costa, Prajwal Kafle, Sanjib Sharma, and Gayandhi M. De SilvaNatur, 2020Globular clusters are some of the oldest bound stellar structures observed in the Universe1. They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the hierarchical build-up of structure2,3. Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a āmetallicity floorā, whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun4-6. The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to formāboth critical components for understanding the build-up of mass in the Universe7. Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance ([Fe/H] = -2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution.
K2-HERMES II. Planet-candidate properties from K2 Campaigns 1-13
Robert A. Wittenmyer, Jake T. Clark, Sanjib Sharma, Dennis Stello, Jonathan Horner, Stephen R. Kane, Catherine P. Stevens, Duncan J. Wright, Lorenzo Spina, Klemen Äotar, Martin Asplund, Joss Bland-Hawthorn, Sven Buder, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken Freeman, Janez Kos, Geraint Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Jeffrey D. Simpson, Daniel B. Zucker, and Tomaz ZwitterMNRAS, 2020Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present complete results for planet-candidate hosts from the K2-HERMES survey, which uses the HERMES multi-object spectrograph on the Anglo-Australian Telescope to obtain Rā¼28ā000 spectra for more than 30 000 K2 stars. We present complete host-star parameters and planet-candidate radii for 224 K2 candidate planets from C1-C13. Our results cast severe doubt on 30 K2 candidates, as we derive unphysically large radii, larger than 2RJup. This work highlights the importance of obtaining accurate, precise, and self-consistent stellar parameters for ongoing large planet search programs - something that will only become more important in the coming years, as TESS begins to deliver its own harvest of exoplanets.
Abundances in the Milky Way across Five Nucleosynthetic Channels from 4 Million LAMOST Stars
Adam Wheeler, Melissa Ness, Sven Buder, Joss Bland-Hawthorn, Gayandhi De Silva, Michael Hayden, Janez Kos, Geraint F. Lewis, Sarah Martell, Sanjib Sharma, Jeffrey D. Simpson, D. B. Zucker, and Tomaž ZwitterApJ, 2020Large stellar surveys are revealing the chemodynamical structure of the Galaxy across a vast spatial extent. However, the many millions of low-resolution spectra observed to date are yet to be fully exploited. We employ The Cannon, a data-driven approach for estimating chemical abundances, to obtain detailed abundances from low-resolution (R = 1800) LAMOST spectra, using the GALAH survey as our reference. We deliver five (for dwarfs) or six (for giants) estimated abundances representing five different nucleosynthetic channels, for 3.9 million stars, to a precision of 0.05-0.23 dex. Using wide binary pairs, we demonstrate that our abundance estimates provide chemical discriminating power beyond metallicity alone. We show the coverage of our catalog with radial, azimuthal and dynamical abundance maps and examine the neutron capture abundances across the disk and halo, which indicate different origins for the in situ and accreted halo populations. LAMOST has near-complete Gaia coverage and provides an unprecedented perspective on chemistry across the Milky Way.
The closest extremely low-mass white dwarf to the Sun
Adela Kawka, Jeffrey D. Simpson, StĆ©phane Vennes, Michael S. Bessell, Gary S. Da Costa, Anna F. Marino, and Simon J. MurphyMNRAS, 2020We present the orbit and properties of 2MASS J050051.85-093054.9, establishing it as the closest (d ā 71 pc) extremely low-mass white dwarf to the Sun. We find that this star is hydrogen rich with T_\textrm effā10ā500 K, log g ā 5.9, and, following evolutionary models, has a mass of ā0.17 Mā. Independent analysis of radial velocity and Transiting Exoplanet Survey Satellite(TESS) photometric time series reveals an orbital period of ā9.5 h. Its high velocity amplitude ( Kā144Ā \textrm kmā\textrm s^-1 ) produces a measurable Doppler beaming effect in the TESSlight curve with an amplitude of 1 mmag. The unseen companion is most likely a faint white dwarf. J0500-0930 belongs to a class of post-common envelope systems that will most likely merge through unstable mass transfer and in specific circumstances lead to Type Ia supernova explosions.
The GALAH survey: multiple stars and our Galaxy. I. A comprehensive method for deriving properties of FGK binary stars
G. Traven, S. Feltzing, T. Merle, M. Swaelmen, K. Äotar, R. Church, T. Zwitter, Y. -S. Ting, C. Sahlholdt, M. Asplund, J. Bland-Hawthorn, G. De Silva, K. Freeman, S. Martell, S. Sharma, D. Zucker, S. Buder, A. Casey, V. DāOrazi, J. Kos, G. Lewis, J. Lin, K. Lind, J. Simpson, D. Stello, U. Munari, and R. A. WittenmyerA&A, 2020Context. Binary stellar systems form a large fraction of the Galaxyās stars. They are useful as laboratories for studying the physical processes taking place within stars, and must be correctly taken into account when observations of stars are used to study the structure and evolution of the Galaxy. The advent of large-scale spectroscopic and photometric surveys allows us to obtain large samples of binaries that permit characterising their populations.
Catalogue is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/638/A145
Aims: We aim to obtain a large sample of double-lined spectroscopic binaries (SB2s) by analysis of spectra from the GALAH survey in combination with photometric and astrometric data. A combined analysis will provide stellar parameters of thousands of binary stars that can be combined to form statistical observables of a given population. We aim to produce a catalogue of well-characterised systems, which can in turn be compared to models of populations of binary stars, or to follow-up individual systems of interest.
Methods: We obtained a list of candidate SB2 systems from a t-distributed stochastic neighbour embedding (t-SNE) classification and a cross-correlation analysis of GALAH spectra. To compute parameters of the primary and secondary star, we used a Bayesian approach that includes a parallax prior from Gaia DR2, spectra from GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. We used a Markov chain Monte Carlo approach to sample the posterior distributions of the following model parameters for the two stars: Teff[1,2], logg[1,2], [Fe/H], Vr[1,2], vmic[1,2], vbroad[1,2], R[1,2], and E(B-V).
Results: We present results for 12 760 binary stars detected as SB2s. We construct the statistical observables T1/T2, ĪVr, and R1/R2, which demonstrate that our sample mostly consists of dwarfs, with a significant fraction of evolved stars and several dozen members of the giant branch. The majority of these binary stars is concentrated at the lower boundary of the ĪVr distribution, and the R1/R2 ratio is mostly close to unity. The derived metallicity of our binary stars is statistically lower than that of single dwarf stars from the same magnitude-limited sample.
Conclusions: Our sample of binary stars represents a large population of well-characterised double-lined spectroscopic binaries that are appropriate for statistical studies of the binary populations. The derived stellar properties and their distributions show trends that are expected for a population of close binary stars (a < 10 AU) detected through double lines in their spectra. Our detection technique allows us to probe binary systems with mass ratios 0.5 ā¤q ā¤ 1.Empirical Relationship between Calcium Triplet Equivalent Widths and [Fe/H] Using Gaia Photometry
Jeffrey D. SimpsonRNAAS, 2020I present a new empirical calibration, using Gaia photometry, of the relationship between the metallicity of red giant branch (RGB) stars and the equivalent widths of their Ca II triplet lines. The equivalent widths are taken from a spectral library of 2050 RGB stars from 18 globular clusters observed with the Anglo-Australian Telescopeās AAOmega spectrograph between 2006 and 2017. The stars cover a metallicity range of -0.49\gt [\mathrmFe/\rmH]\gt -2.27 .
Identifying stellar streams in Gaia DR2 with data mining techniques
Nicholas W. Borsato, Sarah L. Martell, and Jeffrey D. SimpsonMNRAS, 2020Streams of stars from captured dwarf galaxies and dissolved globular clusters are identifiable through the similarity of their orbital parameters, a fact that remains true long after the streams have dispersed spatially. We calculate the integrals of motion for 31 234 stars, to a distance of 4 kpc from the Sun, which have full and accurate 6D phase space positions in the Gaia DR2 catalogue. We then apply a novel combination of data mining, numerical, and statistical techniques to search for stellar streams. This process returns five high confidence streams (including one which was previously undiscovered), all of which display tight clustering in the integral of motion space. Colour-magnitude diagrams indicate that these streams are relatively simple, old, metal-poor populations. One of these resolved streams shares very similar kinematics and metallicity characteristics with the Gaia-Enceladus dwarf galaxy remnant, but with a slightly younger age. The success of this project demonstrates the usefulness of data mining techniques in exploring large data sets.
The Pristine Inner Galaxy Survey (PIGS) I: tracing the kinematics of metal-poor stars in the Galactic bulge
A. Arentsen, E. Starkenburg, N. F. Martin, V. Hill, R. Ibata, A. Kunder, M. Schultheis, K. A. Venn, D. B. Zucker, D. Aguado, R. Carlberg, J. I. GonzĆ”lez HernĆ”ndez, C. Lardo, N. Longeard, K. Malhan, J. F. Navarro, R. SĆ”nchez-Janssen, F. Sestito, G. Thomas, K. Youakim, G. F. Lewis, J. D. Simpson, and Z. WanMNRAS, 2020Our Galaxy is known to contain a central boxy/peanut-shaped bulge, yet the importance of a classical, pressure-supported component within the central part of the Milky Way is still being debated. It should be most visible at low metallicity, a regime that has not yet been studied in detail. Using metallicity-sensitive narrow-band photometry, the Pristine Inner Galaxy Survey (PIGS) has collected a large sample of metal-poor ([Fe/H] < -1.0) stars in the inner Galaxy to address this open question. We use PIGS to trace the metal-poor inner Galaxy kinematics as function of metallicity for the first time. We find that the rotational signal decreases with decreasing [Fe/H] , until it becomes negligible for the most metal-poor stars. Additionally, the velocity dispersion increases with decreasing metallicity for -3.0 < [Fe/H] < -0.5, with a gradient of -44 Ā± 4 km s-1 dex-1. These observations may signal a transition between Galactic components of different metallicities and kinematics, a different mapping on to the boxy/peanut-shaped bulge for former disc stars of different metallicities and/or the secular dynamical and gravitational influence of the bar on the pressure-supported component. Our results provide strong constraints on models that attempt to explain the properties of the inner Galaxy.
The GALAH Survey: Chemically tagging the Fimbulthul stream to the globular cluster Ļ Centauri
Jeffrey D. Simpson, Sarah L. Martell, Gary Da Costa, Jonathan Horner, Rosemary F. G. Wyse, Yuan-Sen Ting, Martin Asplund, Joss Bland-Hawthorn, Sven Buder, Gayandhi M. De Silva, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Karin Lind, Sanjib Sharma, Daniel B. Zucker, Tomaž Zwitter, Klemen Äotar, Peter L. Cottrell, and Thomas NordlanderMNRAS, 2020Using kinematics from Gaia and the large elemental abundance space of the second data release of the GALAH survey, we identify two new members of the Fimbulthul stellar stream, and chemically tag them to massive, multimetallic globular cluster Ļ Centauri. Recent analysis of the second data release of Gaia had revealed the Fimbulthul stellar stream in the halo of the Milky Way. It had been proposed that the stream is associated with the Ļ Cen, but this proposition relied exclusively upon the kinematics and metallicities of the stars to make the association. In this work, we find our two new members of the stream to be metal-poor stars that are enhanced in sodium and aluminium, typical of second population globular cluster stars, but not otherwise seen in field stars. Furthermore, the stars share the s-process abundance pattern seen in Ļ Cen, which is rare in field stars. Apart from one star within 1.5 deg of Ļ Cen, we find no other stars observed by GALAH spatially near Ļ Cen or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to Ļ Cen confirms the earlier work, and further links this tidal feature in the Milky Way halo to Ļ Cen.
Discovery of a nearby 1700 km s-1 star ejected from the Milky Way by Sgr A*
Sergey E. Koposov, Douglas Boubert, Ting S. Li, Denis Erkal, Gary S. Da Costa, Daniel B. Zucker, Alexander P. Ji, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Jeffrey D. Simpson, Nora Shipp, Zhen Wan, Vasily Belokurov, Joss Bland-Hawthorn, Sarah L. Martell, Thomas Nordlander, Andrew B. Pace, Gayandhi M. De Silva, Mei-Yu Wang, and S5 CollaborationMNRAS, 2020We present the serendipitous discovery of the fastest main-sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a ā¼2.35 Mā A-type star located at a distance of ā¼9 kpc from the Sun and has a heliocentric radial velocity of 1017 Ā± 2.7 km s^-1 without any signature of velocity variability. The current 3D velocity of the star in the Galactic frame is 1755 Ā± 50 km s^-1. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of ā¼1800 km s^-1 and travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion Vy,ā = 246.1 Ā± 5.3 km s^-1 or position R0 = 8.12 Ā± 0.23 kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disc of young stars at the Galactic Centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.
The GALAH survey: temporal chemical enrichment of the galactic disc
Jane Lin, Martin Asplund, Yuan-Sen Ting, Luca Casagrande, Sven Buder, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, K. Lind, Sarah L. Martell, Sanjib Sharma, Jeffrey D. Simpson, Tomaž Zwitter, Daniel B. Zucker, Ivan Minchev, Klemen Äotar, Michael Hayden, Jonti Horner, Geraint F. Lewis, Thomas Nordlander, Rosemary F. G. Wyse, and MaruÅ”a ŽerjalMNRAS, 2020We present isochrone ages and initial bulk metallicities ([Fe/H]_bulk, by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants (7000 K> T_ eff> 4000 K and log g>3 dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different [Fe/H]_bulk bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger sample to be compatible with studies based on small (ā¼100 stars) samples of solar twins, but we now extend them to both sub- and supersolar metallicities. The Ī±-elements show differing behaviour: the hydrostatic Ī±-elements O and Mg show a steady decline with time for all metallicities, while the explosive Ī±-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages ā² 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers.
2019
The K2-HERMES Survey: age and metallicity of the thick disc
Sanjib Sharma, Dennis Stello, Joss Bland-Hawthorn, Michael R. Hayden, Joel C. Zinn, Thomas Kallinger, Marc Hon, Martin Asplund, Sven Buder, Gayandhi M. De Silva, Valentina DāOrazi, Ken Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah Martell, Jeffrey D. Simpson, Rob A. Wittenmyer, Daniel B. Zucker, Tomaz Zwitter, Timothy R. Bedding, Boquan Chen, Klemen Cotar, James Esdaile, Jonathan Horner, Daniel Huber, Prajwal R. Kafle, Shourya Khanna, Tanda Li, Yuan-Sen Ting, David M. Nataf, Thomas Nordlander, Mohd Hafiz Mohd Saadon, Gregor Traven, Duncan Wright, and Rosemary F. G. WyseMNRAS, 2019Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the Kepler satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We use an importance-sampling framework, which takes the selection function into account, to fit for the metallicities of a population synthesis model using spectroscopic data. We show that spectroscopic measurements of [Fe/H] and [Ī±/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of log (Z/Zā) = -0.16 for the thick disc. Here Z is the effective solar-scaled metallicity, which is a function of [Fe/H] and [Ī±/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This indirectly verifies that the asteroseismic mass scaling relation is good to within five per cent. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old Ī±-enhanced thick disc.
The southern stellar stream spectroscopic survey (Sāµ): Overview, target selection, data reduction, validation, and early science
T. S. Li, S. E. Koposov, D. B. Zucker, G. F. Lewis, K. Kuehn, J. D. Simpson, A. P. Ji, N. Shipp, Y. -Y. Mao, M. Geha, A. B. Pace, A. D. Mackey, S. Allam, D. L. Tucker, G. S. Da Costa, D. Erkal, J. D. Simon, J. R. Mould, S. L. Martell, Z. Wan, G. M. De Silva, K. Bechtol, E. Balbinot, V. Belokurov, J. Bland-Hawthorn, A. R. Casey, L. Cullinane, A. Drlica-Wagner, S. Sharma, A. K. Vivas, R. H. Wechsler, B. Yanny, and S5 CollaborationMNRAS, 2019We introduce the southern stellar stream spectroscopy survey (S5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of S5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescopeās 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S5 has mapped nine DES streams and three streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S5, including future public data releases, can be found at http://s5collab.github.io.
A Data-driven Model of Nucleosynthesis with Chemical Tagging in a Lower-dimensional Latent Space
Andrew R. Casey, John C. Lattanzio, Aldeida Aleti, David L. Dowe, Joss Bland-Hawthorn, Sven Buder, Geraint F. Lewis, Sarah L. Martell, Thomas Nordlander, Jeffrey D. Simpson, Sanjib Sharma, and Daniel B. ZuckerApJ, 2019Chemical tagging seeks to identify unique star formation sites from present-day stellar abundances. Previous techniques have treated each abundance dimension as being statistically independent, despite theoretical expectations that many elements can be produced by more than one nucleosynthetic process. In this work, we introduce a data-driven model of nucleosynthesis, where a set of latent factors (e.g., nucleosynthetic yields) contribute to all stars with different scores and clustering (e.g., chemical tagging) is modeled by a mixture of multivariate Gaussians in a lower-dimensional latent space. We use an exact method to simultaneously estimate the factor scores for each star, the partial assignment of each star to each cluster, and the latent factors common to all stars, even in the presence of missing data entries. We use an information-theoretic Bayesian principle to estimate the number of latent factors and clusters. Using the second Galah data release, we find that six latent factors are preferred to explain N = 2566 stars with 17 chemical abundances. We identify the rapid- and slow neutron-capture processes, as well as latent factors consistent with Fe-peak and Ī±-element production, and another where K and Zn dominate. When we consider N ā¼ 160,000 stars with missing abundances, we find another seven factors, as well as 16 components in latent space. Despite these components showing separation in chemistry, which is explained through different yield contributions, none show significant structure in their positions or motions. We argue that more data and joint priors on cluster membership that are constrained by dynamical models are necessary to realize chemical tagging at a galactic-scale. We release accompanying software that scales well with the available data, allowing for the modelās parameters to be optimized in seconds given a fixed number of latent factors, components, and ā¼107 abundance measurements.
A nitrogen-enhanced metal-poor star discovered in the globular cluster ESO280-SC06
Jeffrey D. Simpson, and Sarah L. MartellMNRAS, 2019We report the discovery of the only very nitrogen-enhanced metal-poor star known in a Galactic globular cluster. This star, in the very metal-poor cluster ESO280-SC06 , has [N/Fe] > +2.5, while the other stars in the cluster show no obvious enhancement in nitrogen. Around 80 NEMP stars are known in the field, and their abundance patterns are believed to reflect mass transfer from a binary companion in the asymptotic giant branch phase. The dense environment of globular clusters is detrimental to the long term survival of binary systems, resulting in a low observed binary fraction among red giants and the near absence of NEMP stars. We also identify the first known horizontal branch members of ESO280-SC06 , which allow for a much better constraint on its distance. We calculate an updated orbit for the cluster based on our revised distance of 20.6 Ā± 0.5 kpc, and find no significant change to its orbital properties.
The GALAH survey and Gaia DR2: Linking ridges, arches, and vertical waves in the kinematics of the Milky Way
Shourya Khanna, Sanjib Sharma, Thor Tepper-Garcia, Joss Bland-Hawthorn, Michael Hayden, Martin Asplund, Sven Buder, Boquan Chen, Gayandhi M. De Silva, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Sarah L. Martell, Jeffrey D. Simpson, Thomas Nordlander, Dennis Stello, Yuan-Sen Ting, Daniel B. Zucker, and Tomaž ZwitterMNRAS, 2019Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates (R,Ļ ,z), ridge-like structures can be seen in the (R,V_Ļ) plane and asymmetric arch-like structures in the (V_R,V_Ļ) plane. We show that the ridges are also clearly present when the third dimension of the (R,V_Ļ) plane is represented by < z >, < V_z >, < V_R >, <[Fe/H]>, and < [Ī± /Fe]>. The maps suggest that stars along the ridges lie preferentially close to the Galactic mid-plane (|z|< 0.2 kpc), and have metallicity and Ī± elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy - the ridges and arches are simply surfaces of constant energy. We identify eight distinct ridges in the Gaia DR2 data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in < z > and < V_z > suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.
Proper Motions of Stellar Streams Discovered in the Dark Energy Survey
N. Shipp, T. S. Li, A. B. Pace, D. Erkal, A. Drlica-Wagner, B. Yanny, V. Belokurov, W. Wester, S. E. Koposov, K. Kuehn, G. F. Lewis, J. D. Simpson, Z. Wan, D. B. Zucker, S. L. Martell, M. Y. Wang, and S5 CollaborationApJ, 2019We cross-match high-precision astrometric data from Gaia DR2 with accurate multiband photometry from the Dark Energy Survey (DES) DR1 to confidently measure proper motions for nine stellar streams in the DES footprint: Aliqa Uma, ATLAS, Chenab, Elqui, Indus, Jhelum, Phoenix, Tucana III, and Turranburra. We determine low-confidence proper-motion measurements for four additional stellar streams: Ravi, Wambelong, Willka Yaku, and Turbio. We find evidence for a misalignment between stream tracks and the systemic proper motion of streams that may suggest a systematic gravitational influence from the Large Magellanic Cloud (LMC). These proper motions, when combined with radial velocity measurements, will allow for detailed orbit modeling that can be used to constrain properties of the LMC and its effect on nearby streams, as well as global properties of the Milky Wayās gravitational potential.
Discovery of a 21 Myr old stellar population in the Orion complexā
Janez Kos, Joss Bland-Hawthorn, Martin Asplund, Sven Buder, Geraint F. Lewis, Jane Lin, Sarah L. Martell, Melissa K. Ness, Sanjib Sharma, Gayandhi M. De Silva, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Klemen Äotar, and Lorenzo SpinaA&A, 2019Context. The Orion complex is arguably the most studied star-forming region in the Galaxy. While stars are still being born in the Orion nebula, the oldest part was believed to be no more than 13 Myr old.
Movie associated to Fig. 3 is available at https://www.aanda.org Tables B.1-B.5 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A166
Aims: In order to study the full hierarchy of star formation across the Orion complex, we perform a clustering analysis of the Ori OB1a region using new stellar surveys and derive robust ages for each identified stellar aggregate.
Methods: We use Gaia DR2 parameters supplemented with radial velocities from the GALAH and APOGEE surveys to perform clustering of the Ori OB1a association. Five overdensities are resolved in a six-dimensional parameter space (positions, distance, proper motions, and radial velocity). Most correspond to previously known structures (ASCC 16, 25 Orionis, ASCC 20, ASCC 21). We use Gaia DR2, Pan-STARRS1 and 2MASS photometry to fit isochrones to the colour-magnitude diagrams of the identified clusters. The ages of the clusters can thus be measured with ā¼10% precision.
Results: While four of the clusters have ages between 11 and 13 Myr, the ASCC 20 cluster stands out at an age of 21 Ā± 3 Myr. This is significantly greater than the age of any previously known component of the Orion complex. To some degree, all clusters overlap in at least one of the six phase-space dimensions.
Conclusions: We argue that the formation history of the Orion complex, and its relation to the Gould belt, must be reconsidered. A significant challenge in reconstructing the history of the Ori OB1a association is to understand the impact of the newly discovered 21 Myr old population on the younger parts of the complex, including their formation.The retrograde orbit of the globular cluster FSR1758 revealed with Gaia DR2
Jeffrey D. SimpsonMNRAS, 2019We report the first radial velocity measurements of the recently identified globular cluster FSR1758. From the two member stars with radial velocities from the Gaia Radial Velocity Spectrograph reported in Gaia DR2, we find FSR1758 has a radial velocity of 227 Ā± 1 km s^-1. We also find potential extra-tidal star lost from the cluster in the surrounding 1 deg. Combined with Gaia proper motions and photometric distance estimates, this shows that FSR1758 is on a relatively retrograde, radial orbit with a pericentre of 3.8_-0.9^+0.9 kpc, an apocentre of 16_-5^+8 kpc, and eccentricity of 0.62_-0.04^+0.05. Although it is currently at a Galactocentric distance of 3.8_-0.9^+0.9 kpc - at the edge of the bulge - it is an intruder from the halo. We investigate whether a reported āhaloā of stars around FSR1758 is related to the cluster, and find that most of these stars are likely foreground dwarf stars. We conclude that FSR1758 is not a dwarf galaxy, but rather a globular cluster.
The GALAH survey: unresolved triple Sun-like stars discovered by the Gaia mission
Klemen Äotar, Tomaž Zwitter, Gregor Traven, Janez Kos, Martin Asplund, Joss Bland-Hawthorn, Sven Buder, Valentina DāOrazi, Gayandhi M. Silva, Jane Lin, Sarah L. Martell, Sanjib Sharma, Jeffrey D. Simpson, Daniel B. Zucker, Jonathan Horner, Geraint F. Lewis, Thomas Nordlander, Yuan-Sen Ting, Rob A. Wittenmyer, and GALAH CollaborationMNRAS, 2019The latest Gaia data release enables us to accurately identify stars that are more luminous than would be expected on the basis of their spectral type and distance. During an investigation of the 329 best solar twin candidates uncovered among the spectra acquired by the GALAH survey, we identified 64 such overluminous stars. In order to investigate their exact composition, we developed a data-driven methodology that can generate a synthetic photometric signature and spectrum of a single star. By combining multiple such synthetic stars into an unresolved binary or triple system and comparing the results to the actual photometric and spectroscopic observations, we uncovered 6 definitive triple stellar system candidates and an additional 14 potential candidates whose combined spectrum mimics the solar spectrum. Considering the volume correction factor for a magnitude-limited survey, the fraction of probable unresolved triple stars with long orbital periods is ā¼2 per cent. Possible orbital configurations of the candidates were investigated using the selection and observational limits. To validate the discovered multiplicity fraction, the same procedure was used to evaluate the multiplicity fraction of other stellar types.
The GALAH survey and Gaia DR2: dissecting the stellar discās phase space by age, action, chemistry, and location
Joss Bland-Hawthorn, Sanjib Sharma, Thor Tepper-Garcia, James Binney, Ken C. Freeman, Michael R. Hayden, Janez Kos, Gayandhi M. De Silva, Simon Ellis, Geraint F. Lewis, Martin Asplund, Sven Buder, Andrew R. Casey, Valentina DāOrazi, Ly Duong, Shourya Khanna, Jane Lin, Karin Lind, Sarah L. Martell, Melissa K. Ness, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Prajwal R. Kafle, Alice C. Quillen, Yuan-Sen Ting, and Rosemary F. G. WyseMNRAS, 2019We use the second data releases of the European Space AgencyGaia astrometric survey and the high-resolution Galactic Archaeology with HERMES (GALAH) spectroscopic survey to analyse the structure of our Galaxyās disc components. With GALAH, we separate the Ī±-rich and Ī±-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zV z phase plane, for both VĻ and VR, and recover the remarkable āphase spiralā discovered by Gaia. We identify the anticipated quadrupole signature in zV z of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the Ī±-poor disc, grows in z-amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zV z phase plane lasts ā³ 2 Gyr, but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited ā² 0.5 Gyr ago by an object like Sgr with total mass ā¼3 Ć 1010 Mā (stripped down from ā¼5 Ć 1010 Mā when it first entered the halo) passing through the plane.
The GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS
S. Buder, K. Lind, M. K. Ness, M. Asplund, L. Duong, J. Lin, J. Kos, L. Casagrande, A. R. Casey, J. Bland-Hawthorn, G. M. Silva, V. DāOrazi, K. C. Freeman, S. L. Martell, K. J. Schlesinger, S. Sharma, J. D. Simpson, D. B. Zucker, T. Zwitter, K. Äotar, A. Dotter, M. R. Hayden, E. A. Hyde, P. R. Kafle, G. F. Lewis, D. M. Nataf, T. Nordlander, W. Reid, H. -W. Rix, Ć. SkĆŗladĆ³ttir, D. Stello, Y. -S. Ting, G. Traven, R. F. G. Wyse, and GALAH CollaborationA&A, 2019The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and Gaia provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code SPECTROSCOPY MADE EASY. We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g. Focusing our investigation on the correlations between stellar age, iron abundance [Fe/H], and mean alpha-enhancement [Ī±/Fe] of the magnitude-selected sample, we recover the result that stars of the high-Ī± sequence are typically older than stars in the low-Ī± sequence, the latter spanning iron abundances of -0.7 < [Fe/H] < +0.5. While these two sequences become indistinguishable in [Ī±/Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high-Ī± and the low-Ī± sequence even in this regime. When dissecting the sample by stellar age, we find that the old stars (>8 Gyr) have lower angular momenta Lz than the Sun, which implies that they are on eccentric orbits and originate from the inner disc. Contrary to some previous smaller scale studies we find a continuous evolution in the high-Ī±-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate "high-Ī± metal-rich" population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low Ī±-sequence and show a gradient in Lz from low [Fe/H] (Lz > Lz, ā) towards higher [Fe/H] (Lz < Lz, ā), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.
The catalogue is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A19The GALAH survey: co-orbiting stars and chemical tagging
Jeffrey D. Simpson, Sarah L. Martell, Gary Da Costa, Andrew R. Casey, Ken C. Freeman, Jonathan Horner, Yuan-Sen Ting, David M. Nataf, Geraint F. Lewis, Melissa K. Ness, Daniel B. Zucker, Peter L. Cottrell, Klemen Äotar, Martin Asplund, Joss Bland-Hawthorn, Sven Buder, Valentina DāOrazi, Gayandhi M. De Silva, Ly Duong, Janez Kos, Jane Lin, Karin Lind, Katharine J. Schlesinger, Sanjib Sharma, Tomaž Zwitter, Prajwal R. Kafle, and Thomas NordlanderMNRAS, 2019We present a study using the second data release of the GALAH survey of stellar parameters and elemental abundances of 15 pairs of stars identified by Oh et al. They identified these pairs as potentially co-moving pairs using proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1 pc) pairs of stars do in fact have similar Galactic orbits, while a further four claimed co-moving pairs are not truly co-orbiting. Eight of the 11 co-orbiting pairs have reliable stellar parameters and abundances, and we find that three of those are quite similar in their abundance patterns, while five have significant [Fe/H] differences. For the latter, this indicates that they could be co-orbiting because of the general dynamical coldness of the thin disc, or perhaps resonances induced by the Galaxy, rather than a shared formation site. Stars such as these, wide binaries, debris of past star formation episodes, and coincidental co-orbiters, are crucial for exploring the limits of chemical tagging in the Milky Way.
The GALAH survey: velocity fluctuations in the Milky Way using Red Clump giants
Shourya Khanna, Sanjib Sharma, Joss Bland-Hawthorn, Michael Hayden, David M. Nataf, Yuan-Sen Ting, Janez Kos, Sarah Martell, Tomaž Zwitter, Gayandhi De Silva, Martin Asplund, Sven Buder, Ly Duong, Jane Lin, Jeffrey D. Simpson, Borja Anguiano, Jonathan Horner, Prajwal R. Kafle, Geraint F. Lewis, Thomas Nordlander, Rosemary F. G. Wyse, Robert A. Wittenmyer, and Daniel B. ZuckerMNRAS, 2019If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. Recent results using the APOGEE survey find significant fluctuations in velocity for stars in the mid-plane (|z| <0.25 kpc) out to 5 kpc, suggesting that the dynamical influence of non-axisymmetric features, i.e. the Milky Wayās bar, spiral arms, and merger events extends out to the Solar neighbourhood. Their measured power spectrum has a characteristic amplitude of 11 km s-1 on a scale of 2.5 kpc. The existence of such large-scale streaming motions has important implications for determining the Sunās motion about the Galactic Centre. Using Red Clump stars from GALAH and APOGEE, we map the line-of-sight velocities around the Sun (d < 5 kpc), and |z| <1.25 kpc from the mid-plane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual fluctuations and compare our findings with mock survey generated by GALAXIA. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier APOGEE power spectrum but with 20 per cent smaller amplitude (9.3 km s-1) after taking into account a few systematics (e.g. volume completeness). Using a flexible axisymmetric model the power amplitude is further reduced to 6.3 km s-1. Additionally, our simulations show that, in the plane, distances are underestimated for high-mass Red Clump stars which can lead to spurious power amplitude of about 5.2 km s-1. Taking this into account, we estimate the amplitude of real fluctuations to be <4.6 km s-1, about a factor of three less than the APOGEE result.
2018
The GALAH survey: verifying abundance trends in the open cluster M67 using non-LTE modelling
Xudong Gao, Karin Lind, Anish M. Amarsi, Sven Buder, Aaron Dotter, Thomas Nordlander, Martin Asplund, Joss Bland-Hawthorn, Gayandhi M. Silva, Valentina DāOrazi, Ken C. Freeman, Janez Kos, Geraint F. Lewis, Jane Lin, Sarah L. Martell, Katharine J. Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Gary Costa, Borja Anguiano, Jonathan Horner, Elaina A. Hyde, Prajwal R. Kafle, David M. Nataf, Warren Reid, Dennis Stello, Yuan-Sen Ting, and GALAH CollaborationMNRAS, 2018Open cluster members are coeval and share the same initial bulk chemical composition. Consequently, differences in surface abundances between members of a cluster that are at different evolutionary stages can be used to study the effects of mixing and internal chemical processing. We carry out an abundance analysis of seven elements (Li, O, Na, Mg, Al, Si, and Fe) in 66 stars belonging to the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model atmospheres, and non-local thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE analysis, we find a typical star-to-star scatter in the abundance ratios of around 0.05 dex. We find trends in the abundance ratios with effective temperature, indicating systematic differences in the surface abundances between turn-off and giant stars; these trends are more pronounced when LTE is assumed. However, trends with effective temperature remain significant for Al and Si also in non-LTE. Finally, we compare the derived abundances with prediction from stellar evolution models including effects of atomic diffusion. We find overall good agreement for the abundance patterns of dwarfs and sub-giant stars, but the abundances of cool giants are lower relative to less evolved stars than predicted by the diffusion models, in particular for Mg.
The GALAH survey: accurate radial velocities and library of observed stellar template spectra
Tomaž Zwitter, Janez Kos, Andrea Chiavassa, Sven Buder, Gregor Traven, Klemen Äotar, Jane Lin, Martin Asplund, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi De Silva, Ly Duong, Kenneth C. Freeman, Karin Lind, Sarah Martell, Valentina DāOrazi, Katharine J. Schlesinger, Jeffrey D. Simpson, Sanjib Sharma, Daniel B. Zucker, Borja Anguiano, Luca Casagrande, Remo Collet, Jonathan Horner, Michael J. Ireland, Prajwal R. Kafle, Geraint Lewis, Ulisse Munari, David M. Nataf, Melissa Ness, Thomas Nordlander, Dennis Stello, Yuan-Sen Ting, Chris G. Tinney, Fred Watson, Rob A. Wittenmyer, and MaruÅ”a ŽerjalMNRAS, 2018GALAH is a large-scale magnitude-limited southern stellar spectroscopic survey. Its second data release (GALAH DR2) provides values of stellar parameters and abundances of 23 elements for 342 682 stars (Buder et al.). Here we add a description of the public release of radial velocities with a typical accuracy of 0.1 km s-1 for 336 215 of these stars, achievable due to the large wavelength coverage, high resolving power, and good signal-to-noise ratio of the observed spectra, but also because convective motions in stellar atmosphere and gravitational redshift from the star to the observer are taken into account. In the process we derive medians of observed spectra that are nearly noiseless, as they are obtained from between 100 and 1116 observed spectra belonging to the same bin with a width of 50 K in temperature, 0.2 dex in gravity, and 0.1 dex in metallicity. Publicly released 1181 median spectra have a resolving power of 28 000 and trace the well-populated stellar types with metallicities between -0.6 and +0.3. Note that radial velocities from GALAH are an excellent match to the accuracy of velocity components along the sky plane derived by Gaia for the same stars. The level of accuracy achieved here is adequate for studies of dynamics within stellar clusters, associations, and streams in the Galaxy. So it may be relevant for studies of the distribution of dark matter.
Holistic spectroscopy: complete reconstruction of a wide-field, multiobject spectroscopic image using a photonic comb
Janez Kos, Joss Bland-Hawthorn, Christopher H. Betters, Sergio Leon-Saval, Martin Asplund, Sven Buder, Andrew R. Casey, Valentina DāOrazi, Gayandhi Silva, Ken Freeman, Geraint Lewis, Jane Lin, Sarah L. Martell, Katharine Schlesinger, Sanjib Sharma, Jeffrey D. Simpson, Daniel Zucker, Tomaž Zwitter, Michael Hayden, Jonathan Horner, David M. Nataf, and Yuan-Sen TingMNRAS, 2018The primary goal of Galactic archaeology is to learn about the origin of the Milky Way from the detailed chemistry and kinematics of millions of stars. Wide-field multifibre spectrographs are increasingly used to obtain spectral information for huge samples of stars. Some surveys (e.g. GALAH) are attempting to measure up to 30 separate elements per star. Stellar abundance spectroscopy is a subtle art that requires a very high degree of spectral uniformity across each of the fibres. However, wide-field spectrographs are notoriously non-uniform due to the fast output optics necessary to image many fibre outputs on to the detector. We show that precise spectroscopy is possible with such instruments across all fibres by employing a photonic comb - a device that produces uniformly spaced spots of light on the CCD to precisely map complex aberrations. Aberrations are parametrized by a set of orthogonal moments with ā¼100 independent parameters. We then reproduce the observed image by convolving high-resolution spectral templates with measured aberrations as opposed to extracting the spectra from the observed image. Such a forward modelling approach also trivializes some spectroscopic reduction problems like fibre cross-talk, and reliably extracts spectra with a resolution ā¼2.3 times above the nominal resolution of the instrument. Our rigorous treatment of optical aberrations also encourages a less conservative spectrograph design in the future.
The GALAH survey and Gaia DR2: (non-)existence of five sparse high-latitude open clusters
Janez Kos, Gayandhi Silva, Sven Buder, Joss Bland-Hawthorn, Sanjib Sharma, Martin Asplund, Valentina DāOrazi, Ly Duong, Ken Freeman, Geraint F. Lewis, Jane Lin, Karin Lind, Sarah L. Martell, Katharine J. Schlesinger, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Timothy R. Bedding, Klemen Äotar, Jonathan Horner, Thomas Nordlander, Denis Stello, Yuan-Sen Ting, and Gregor TravenMNRAS, 2018Sparse open clusters can be found at high galactic latitudes where loosely populated clusters are more easily detected against the lower stellar background. Because most star formation takes place in the thin disc, the observed population of clusters far from the Galactic plane is hard to explain. We combined spectral parameters from the GALAH survey with the Gaia DR2 catalogue to study the dynamics and chemistry of five old sparse high-latitude clusters in more detail. We find that four of them (NGC 1252, NGC 6994, NGC 7772, NGC 7826) - originally classified in 1888 - are not clusters but are instead chance projections on the sky. Member stars quoted in the literature for these four clusters are unrelated in our multidimensional physical parameter space; the quoted cluster properties in the literature are therefore meaningless. We confirm the existence of visually similar NGC 1901 for which we provide a probabilistic membership analysis. An overdensity in three spatial dimensions proves to be enough to reliably detect sparse clusters, but the whole six-dimensional space must be used to identify members with high confidence, as demonstrated in the case of NGC 1901.
The GALAH Survey: second data release
Sven Buder, Martin Asplund, Ly Duong, Janez Kos, Karin Lind, Melissa K. Ness, Sanjib Sharma, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. Silva, Valentina DāOrazi, Ken C. Freeman, Geraint F. Lewis, Jane Lin, Sarah L. Martell, Katharine J. Schlesinger, Jeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Anish M. Amarsi, Borja Anguiano, Daniela Carollo, Luca Casagrande, Klemen Äotar, Peter L. Cottrell, Gary Costa, Xudong D. Gao, Michael R. Hayden, Jonathan Horner, Michael J. Ireland, Prajwal R. Kafle, Ulisse Munari, David M. Nataf, Thomas Nordlander, Dennis Stello, Yuan-Sen Ting, Gregor Traven, Fred Watson, Robert A. Wittenmyer, Rosemary F. G. Wyse, David Yong, Joel C. Zinn, MaruÅ”a Žerjal, and GALAH CollaborationMNRAS, 2018The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way, designed to deliver complementary chemical information to a large number of stars covered by the Gaia mission. We present the GALAH second public data release (GALAH DR2) containing 342 682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction, and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multistep approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels (Teff, log g, [Fe/H], [X/Fe], vmic, vsin i, A_K_S) for a representative training set of stars. This information is then propagated to the whole sample with the data-driven method of The Cannon. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D MARCS stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence to our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from Gaia will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.
The GALAH survey: stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere, and metallicity
Alice C. Quillen, Gayandhi De Silva, Sanjib Sharma, Michael Hayden, Ken Freeman, Joss Bland-Hawthorn, MaruÅ”a Žerjal, Martin Asplund, Sven Buder, Valentina DāOrazi, Ly Duong, Janez Kos, Jane Lin, Karin Lind, Sarah Martell, Katharine Schlesinger, Jeffrey D. Simpson, Daniel B. Zucker, Tomaz Zwitter, Borja Anguiano, Daniela Carollo, Luca Casagrande, Klemen Cotar, Peter L. Cottrell, Michael Ireland, Prajwal R. Kafle, Jonathan Horner, Geraint F. Lewis, David M. Nataf, Yuan-Sen Ting, Fred Watson, Rob Wittenmyer, and Rosemary WyseMNRAS, 2018Using GALAH (GALactic Archaeology with HERMES) survey data of nearby stars, we look at how structure in the planar (u, v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars with distance d ā² 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H] > 0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km s-1 kpc. The association of the gap with a particular angular momentum value supports a bar resonant model for the Hercules stream. Moving groups previously identified in Hipparcos(HIgh Precision Parallax COllecting Satellite) observations are easiest to see in stars nearer than 250 pc, and their visibility and peak velocities in the velocity distributions depends on both viewing direction (galactic longitude and hemisphere) and metallicity. We infer that there is fine structure in local velocity distributions that varies over distances of a few hundred pc in the Galaxy.
The most metal-poor Galactic globular cluster: the first spectroscopic observations of ESO280-SC06
Jeffrey D. SimpsonMNRAS, 2018We present the first spectroscopic observations of the very metal-poor Milky Way globular cluster ESO280-SC06. Using spectra acquired with the 2dF/AAOmega spectrograph on the Anglo-Australian Telescope, we have identified 13 members of the cluster, and estimate from their infrared calcium triplet lines that the cluster has a metallicity of [Fe/H]=-2.48^+0.06 _ -0.11. This would make it the most metal-poor globular cluster known in the Milky Way. This result was verified with comparisons to three other metal-poor globular clusters that had been observed and analysed in the same manner. We also present new photometry of the cluster from EFOSC2 and SkyMapper and confirm that the cluster is located 22.9 Ā± 2.1 kpc from the Sun and 15.2 Ā± 2.1 kpc from the Galactic Centre, and has a radial velocity of 92.5^+2.4 _ -1.6 km s-1. These new data finds the cluster to have a radius about half that previously estimated, and we find that the cluster has a dynamical mass of the cluster of (12 Ā± 2) Ć 103 Mā. Unfortunately, we lack reliable proper motions to fully characterize its orbit about the Galaxy. Intriguingly, the photometry suggests that the cluster lacks a well-populated horizontal branch, something that has not been observed in a cluster so ancient or metal poor.
The GALAH survey: properties of the Galactic disc(s) in the solar neighbourhood
L. Duong, K. C. Freeman, M. Asplund, L. Casagrande, S. Buder, K. Lind, M. Ness, J. Bland-Hawthorn, G. M. De Silva, V. DāOrazi, J. Kos, G. F. Lewis, J. Lin, S. L. Martell, K. Schlesinger, S. Sharma, J. D. Simpson, D. B. Zucker, T. Zwitter, B. Anguiano, G. S. Da Costa, E. Hyde, J. Horner, P. R. Kafle, D. M. Nataf, W. Reid, D. Stello, Y. -S. Ting, and R. F. G. WyseMNRAS, 2018Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick discs near the solar neighbourhood. The data cover a small range of Galactocentric radius (7.9 ā² R_GC ā² 9.5 kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude 260Ā° ā¤ ā ā¤ 280Ā°). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined āthickā and āthinā discs of the Galaxy. The thin disc (low-Ī± population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/dz = -0.18 Ā± 0.01 dex kpc-1, which is broadly consistent with previous studies. In contrast, its vertical Ī±-abundance profile is almost flat, with a gradient of d[Ī±/M]/dz = 0.008 Ā± 0.002 dex kpc-1. The steep vertical metallicity gradient of the low-Ī± population is in agreement with models where radial migration has a major role in the evolution of the thin disc. The thick disc (high-Ī± population) has a weaker vertical metallicity gradient d[M/H]/dz = -0.058 Ā± 0.003 dex kpc-1. The Ī±-abundance of the thick disc is nearly constant with height, d[Ī±/M]/dz = 0.007 Ā± 0.002 dex kpc-1. The negative gradient in metallicity and the small gradient in [Ī±/M] indicate that the high-Ī± population experienced a settling phase, but also formed prior to the onset of major Type Ia supernova enrichment. We explore the implications of the distinct Ī±-enrichments and narrow [Ī±/M] range of the sub-populations in the context of thick disc formation.
The GALAH and TESS-HERMES surveys: high-resolution spectroscopy of luminous supergiants in the Magellanic Clouds and Bridge
Jeffrey D. Simpson, Dennis Stello, Sanjib Sharma, Yuan-Sen Ting, David M. Nataf, Gary Da Costa, Robert A. Wittenmyer, Jonathan Horner, Sarah L. Martell, Geraint F. Lewis, Gayandhi M. De Silva, Peter L. Cottrell, Martin Asplund, Joss Bland-Hawthorn, Sven Buder, Valentina DāOrazi, Ly Duong, Ken C. Freeman, Janez Kos, Jane Lin, Karin Lind, Katharine. J. Schlesinger, Daniel B. Zucker, Tomaž Zwitter, Prajwal R. Kafle, Shourya Khanna, and Thomas Nordlander2018We report the serendipitous observations of 571 luminous supergiants in the Magellanic Clouds by the spectroscopic GALAH and TESS-HERMES surveys: 434 stars in the Large Magellanic Cloud and 137 in the Small Magellanic Cloud. We also find one star that appears associated with structured star formation in the Magellanic Bridge. Both of these surveys are aimed at the local volume of the Galaxy but have simple, magnitude-limited selection functions that mean they include some observations of luminous extra-Galactic stars. The surveys determine stellar parameter and abundances using The Cannon, a data-driven generative modelling approach. In this work, we explore the results from The Cannon when it is fed the spectra of these intrinsically luminous supergiants in the Magellanic Clouds, which are well outside the normal bounds of The Cannonās training set. We find that, although the parameters are astrophysically incorrect, the v\sin i and the abundances of lithium, barium, and magnesium are excellent discriminants of these stars. It shows that in the future, with an expanded training set, it should be possible to determine accurate values for these types of stars.
The GALAH survey: chemical tagging of star clusters and new members in the Pleiades
Janez Kos, Joss Bland-Hawthorn, Ken Freeman, Sven Buder, Gregor Traven, Gayandhi M. De Silva, Sanjib Sharma, Martin Asplund, Ly Duong, Jane Lin, Karin Lind, Sarah Martell, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Borja Anguiano, Gary Da Costa, Valentina DāOrazi, Jonathan Horner, Prajwal R. Kafle, Geraint Lewis, Ulisse Munari, David M. Nataf, Melissa Ness, Warren Reid, Katie Schlesinger, Yuan-Sen Ting, and Rosemary WyseMNRAS, 2018The technique of chemical tagging uses the elemental abundances of stellar atmospheres to āreconstructā chemically homogeneous star clusters that have long since dispersed. The GALAH spectroscopic survey - which aims to observe one million stars using the Anglo-Australian Telescope - allows us to measure up to 30 elements or dimensions in the stellar chemical abundance space, many of which are not independent. How to find clustering reliably in a noisy high-dimensional space is a difficult problem that remains largely unsolved. Here, we explore t-distributed stochastic neighbour embedding (t-SNE) - which identifies an optimal mapping of a high-dimensional space into fewer dimensions - whilst conserving the original clustering information. Typically, the projection is made to a 2D space to aid recognition of clusters by eye. We show that this method is a reliable tool for chemical tagging because it can: (i) resolve clustering in chemical space alone, (ii) recover known open and globular clusters with high efficiency and low contamination, and (iii) relate field stars to known clusters. t-SNE also provides a useful visualization of a high-dimensional space. We demonstrate the method on a data set of 13 abundances measured in the spectra of 187 000 stars by the GALAH survey. We recover seven of the nine observed clusters (six globular and three open clusters) in chemical space with minimal contamination from field stars and low numbers of outliers. With chemical tagging, we also identify two Pleiades supercluster members (which we confirm kinematically), one as far as 6Ā° - one tidal radius away from the cluster centre.
The K2-HERMES Survey. I. Planet-candidate Properties from K2 Campaigns 1-3
Robert A. Wittenmyer, Sanjib Sharma, Dennis Stello, Sven Buder, Janez Kos, Martin Asplund, Ly Duong, Jane Lin, Karin Lind, Melissa Ness, Tomaz Zwitter, Jonathan Horner, Jake Clark, Stephen R. Kane, Daniel Huber, Joss Bland-Hawthorn, Andrew R. Casey, Gayandhi M. De Silva, Valentina DāOrazi, Ken Freeman, Sarah Martell, Jeffrey D. Simpson, Daniel B. Zucker, Borja Anguiano, Luca Casagrande, James Esdaile, Marc Hon, Michael Ireland, Prajwal R. Kafle, Shourya Khanna, J. P. Marshall, Mohd Hafiz Mohd Saddon, Gregor Traven, and Duncan WrightAJ, 2018Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present first results from the K2-HERMES project, which uses the HERMES multi-object spectrograph on the Anglo-Australian Telescope to obtain R ā¼ 28000 spectra of up to 360 stars in one exposure. This ongoing project aims to derive self-consistent spectroscopic parameters for about half of K2 target stars. We present complete stellar parameters and isochrone-derived masses and radii for 46 stars hosting 57 K2 candidate planets in Campaigns 1-3. Our revised host-star radii cast severe doubt on three candidate planets: EPIC 201407812.01, EPIC 203070421.01, and EPIC 202843107.01, all of which now have inferred radii well in excess of the largest known inflated Jovian planets.
The TESS-HERMES survey data release 1: high-resolution spectroscopy of the TESS southern continuous viewing zone
Sanjib Sharma, Dennis Stello, Sven Buder, Janez Kos, Joss Bland-Hawthorn, Martin Asplund, Ly Duong, Jane Lin, Karin Lind, Melissa Ness, Daniel Huber, Tomaz Zwitter, Gregor Traven, Marc Hon, Prajwal R. Kafle, Shourya Khanna, Hafiz Saddon, Borja Anguiano, Andrew R. Casey, Ken Freeman, Sarah Martell, Gayandhi M. De Silva, Jeffrey D. Simpson, Rob A. Wittenmyer, and Daniel B. ZuckerMNRAS, 2018The Transiting Exoplanet Survey Satellite (TESS) will provide high-precision time series photometry for millions of stars with at least a half-hour cadence. Of particular interest are the circular regions of 12Ā° radius centred around the ecliptic poles that will be observed continuously for a full year. Spectroscopic stellar parameters are desirable to characterize and select suitable targets for TESS, whether they are focused on exploring exoplanets, stellar astrophysics or Galactic archaeology. Here, we present spectroscopic stellar parameters (Teff, log g, [Fe/H], v sin i, vmicro) for about 16 000 dwarf and subgiant stars in TESSā southern continuous viewing zone. For almost all the stars, we also present Bayesian estimates of stellar properties including distance, extinction, mass, radius and age using theoretical isochrones. Stellar surface gravity and radius are made available for an additional set of roughly 8500 red giants. All our target stars are in the range 10 < V < 13.1. Among them, we identify and list 227 stars belonging to the Large Magellanic Cloud. The data were taken using the High Efficiency and Resolution Multi-Element Spectrograph (HERMES; R ā¼ 28 000) at the Anglo-Australian Telescope as part of the TESS-HERMES survey. Comparing our results with the TESS Input Catalogue (TIC) shows that the TIC is generally efficient in separating dwarfs and giants, but it has flagged more than 100 cool dwarfs (Teff < 4800 K) as giants, which ought to be high-priority targets for the exoplanet search. The catalogue can be accessed via http://www.physics.usyd.edu.au/tess-hermes/, or at Mikulski Archive for Space Telescopes (MAST).
2017
ESO 452-SC11: the lowest mass globular cluster with a potential chemical inhomogeneity
Jeffrey D. Simpson, Gayandhi De Silva, Sarah L. Martell, Colin A. Navin, and Daniel B. ZuckerMNRAS, 2017We present the largest spectroscopic investigation of one of the faintest and least studied stellar clusters of the Milky Way, ESO 452-SC11. Using the Anglo-Australian Telescope AAOmega and Keck High Resolution Echelle Spectrometer spectrographs we have identified 11 members of the cluster and found indications of star-to-star light element abundance variation, primarily using the blue cyanogen absorption features. From a stellar density profile, we estimate a total cluster mass of (6.8 Ā± 3.4) Ć 103 Mā. This would make ESO 452-SC11 the lowest mass cluster with evidence for multiple populations. These data were also used to measure the radial velocity of the cluster (16.7 Ā± 0.3 km s-1) and confirm that ESO 452-SC11 is relatively metal rich for a globular cluster ([Fe/H] = -0.81 Ā± 0.13). All known massive clusters studied in detail show multiple populations of stars each with a different chemical composition, but many low-mass globular clusters appear to be chemically homogeneous. ESO 452-SC11 sets a lower mass limit for the multiple stellar population phenomenon.
Siriusly, a newly identified intermediate-age Milky Way stellar cluster: a spectroscopic study of Gaia 1
J. D. Simpson, G. M. De Silva, S. L. Martell, D. B. Zucker, A. M. N. Ferguson, E. J. Bernard, M. Irwin, J. Penarrubia, and E. TolstoyMNRAS, 2017We confirm the reality of the recently discovered Milky Way stellar cluster Gaia 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H]=-0.13Ā± 0.13 and barycentric radial velocity of vr = 58.30 Ā± 0.22 km s-1. These kinematics provide a dynamical mass estimate of 12.9^+4.6_-3.9Ć 10^3 M_ā. Isochrone fits to Gaia, 2MASS, and Pan-STARRS1 photometry indicate that Gaia 1 is an intermediate age (ā¼3 Gyr) stellar cluster. Combining the spatial and kinematic data we calculate Gaia 1 has a circular orbit with a radius of about 12 kpc, but with a large out of plane motion: z_max=1.1^+0.4_-0.3 kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.
The GALAH survey: observational overview and Gaia DR1 companion
S. L. Martell, S. Sharma, S. Buder, L. Duong, K. J. Schlesinger, J. Simpson, K. Lind, M. Ness, J. P. Marshall, M. Asplund, J. Bland-Hawthorn, A. R. Casey, G. De Silva, K. C. Freeman, J. Kos, J. Lin, D. B. Zucker, T. Zwitter, B. Anguiano, C. Bacigalupo, D. Carollo, L. Casagrande, G. S. Da Costa, J. Horner, D. Huber, E. A. Hyde, P. R. Kafle, G. F. Lewis, D. Nataf, C. A. Navin, D. Stello, C. G. Tinney, F. G. Watson, and R. WittenmyerMNRAS, 2017The Galactic Archaeology with HERMES (GALAH) survey is a massive observational project to trace the Milky Wayās history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R ā 28 000) spectra, taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo-Australian Telescope, GALAH will determine stellar parameters and abundances of up to 29 elements for up to one million stars. Selecting targets from a colour-unbiased catalogue built from 2MASS, APASS and UCAC4 data, we expect to observe dwarfs at 0.3-3 kpc and giants at 1-10 kpc. This enables a thorough local chemical inventory of the Galactic thin and thick discs, and also captures smaller samples of the bulge and halo. In this paper, we present the plan, process and progress as of early 2016 for GALAH survey observations. In our first two years of survey observing we have accumulated the largest high-quality spectroscopic data set at this resolution, over 200 000 stars. We also present the first public GALAH data catalogue: stellar parameters (Teff, log(g), [Fe/H], [Ī±/Fe]), radial velocity, distance modulus and reddening for 10 680 observations of 9860 Tycho-2 stars, 7894 of which are included in the first Gaia data release.
A broad perspective on multiple abundance populations in the globular cluster NGC 1851
Jeffrey D. Simpson, Sarah L. Martell, and Colin A. NavinMNRAS, 2017We present an analysis of the multiple stellar populations of the globular cluster NGC 1851. We used lower resolution spectra of giant stars to measure CN, CH, and calcium H & K spectral indices, and determine elemental abundances for carbon and nitrogen. The CN and CH indices were used to confirm that there are four populations of stars in the cluster. The primordial population of stars, with the lowest CN, was found to be generally chemically distinct in elemental abundances from the second-generation populations. As expected, [N/Fe] increases with increasing CN strength, but the only other element that correlated with CN was barium. The two largest populations of stars were found to have the same rate of carbon astration as the stars that ascend the giant branch. We were also able to confirm that four previously identified extratidal stars are chemically associated with the cluster. This work shows the benefit of considering the chemistry of globular clusters with both high- and low-resolution spectra.
The Galah Survey: Classification and Diagnostics with t-SNE Reduction of Spectral Information
G. Traven, G. MatijeviÄ, T. Zwitter, M. Žerjal, J. Kos, M. Asplund, J. Bland-Hawthorn, A. R. Casey, G. De Silva, K. Freeman, J. Lin, S. L. Martell, K. J. Schlesinger, S. Sharma, J. D. Simpson, D. B. Zucker, B. Anguiano, G. Da Costa, L. Duong, J. Horner, E. A. Hyde, P. R. Kafle, U. Munari, D. Nataf, C. A. Navin, W. Reid, and Y. -S. TingApJS, 2017Galah is an ongoing high-resolution spectroscopic survey with the goal of disentangling the formation history of the Milky Way using the fossil remnants of disrupted star formation sites that are now dispersed around the Galaxy. It is targeting a randomly selected magnitude-limited (V ā¤ 14) sample of stars, with the goal of observing one million objects. To date, 300,000 spectra have been obtained. Not all of them are correctly processed by parameter estimation pipelines, and we need to know about them. We present a semi-automated classification scheme that identifies different types of peculiar spectral morphologies in an effort to discover and flag potentially problematic spectra and thus help to preserve the integrity of the survey results. To this end, we employ the recently developed dimensionality reduction technique t-SNE (t-distributed stochastic neighbor embedding), which enables us to represent the complex spectral morphology in a two-dimensional projection map while still preserving the properties of the local neighborhoods of spectra. We find that the majority (178,483) of the 209,533 Galah spectra considered in this study represents normal single stars, whereas 31,050 peculiar and problematic spectra with very diverse spectral features pertaining to 28,579 stars are distributed into 10 classification categories: hot stars, cool metal-poor giants, molecular absorption bands, binary stars, HĪ±/HĪ² emission, HĪ±/HĪ² emission superimposed on absorption, HĪ±/HĪ² P-Cygni, HĪ±/HĪ² inverted P-Cygni, lithium absorption, and problematic. Classified spectra with supplementary information are presented in the catalog, indicating candidates for follow-up observations and population studies of the short-lived phases of stellar evolution.
The GALAH survey: the data reduction pipeline
Janez Kos, Jane Lin, Tomaž Zwitter, MaruÅ”ka Žerjal, Sanjib Sharma, Joss Bland-Hawthorn, Martin Asplund, Andrew R. Casey, Gayandhi M. De Silva, Ken C. Freeman, Sarah L. Martell, Jeffrey D. Simpson, Katharine J. Schlesinger, Daniel Zucker, Borja Anguiano, Carlos Bacigalupo, Timothy R. Bedding, Christopher Betters, Gary Da Costa, Ly Duong, Elaina Hyde, Michael Ireland, Prajwal R. Kafle, Sergio Leon-Saval, Geraint F. Lewis, Ulisse Munari, David Nataf, Dennis Stello, C. G. Tinney, Gregor Traven, Fred Watson, and Robert A. WittenmyerMNRAS, 2017We present the data reduction procedures being used by the GALactic Archeology with Hermes (GALAH) survey, carried out with the HERMES fibre-fed, multi-object spectrograph on the 3.9-m Anglo-Australian Telescope. GALAH is a unique survey, targeting 1 million stars brighter than magnitude V = 14 at a resolution of 28 000 with a goal to measure the abundances of 29 elements. Such a large number of high-resolution spectra necessitate the development of a reduction pipeline optimized for speed, accuracy, and consistency. We outline the design and structure of the IRAF-based reduction pipeline that we developed, specifically for GALAH, to produce fully calibrated spectra aimed for subsequent stellar atmospheric parameter estimation. The pipeline takes advantage of existing IRAF routines and other readily available software so as to be simple to maintain, testable, and reliable. A radial velocity and stellar atmospheric parameter estimator code is also presented, which is used for further data analysis and yields a useful verification of the reduction quality. We have used this estimator to quantify the data quality of GALAH for fibre cross-talk level (ā²0.5 per cent) and scattered light (ā¼5 counts in a typical 20 min exposure), resolution across the field, sky spectrum properties, wavelength solution reliability (better than 1 km s-1 accuracy), and radial velocity precision.
2016
An extreme paucity of second population AGB stars in the ānormalā globular cluster M4
B. T. MacLean, S. W. Campbell, G. M. De Silva, J. Lattanzio, V. DāOrazi, J. D. Simpson, and Y. MomanyMNRAS, 2016Galactic globular clusters (GCs) are now known to harbour multiple stellar populations, which are chemically distinct in many light element abundances. It is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs show different abundance distributions in light elements compared to those in the red giant branch (RGB) and other phases, skewing towards more primordial, field-star-like abundances, which we refer to as subpopulation one (SP1). As part of a larger programme targeting giants in GCs, we obtained high-resolution spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter, we report an extreme paucity of AGB stars with [Na/O] >-0.17 in M4, which contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB abundance distribution is consistent with all AGB stars being from SP1. This result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor) avoid the AGB phase. This is an unexpected result given M4ās horizontal branch morphology - it does not have an extended blue horizontal branch. This is the first abundance study to be performed utilizing the HERMES spectrograph.
The GALAH survey: relative throughputs of the 2dF fibre positioner and the HERMES spectrograph from stellar targets
Jeffrey D. Simpson, G. M. De Silva, J. Bland-Hawthorn, K. C. Freeman, S. L. Martell, Katharine J. Schlesinger, Sanjib Sharma, D. B. Zucker, T. Zwitter, J. Kos, Borja Anguiano, David M. Nataf, Warren Reid, and Robert A. WittenmyerMNRAS, 2016We present an analysis of the relative throughputs of the 3.9-m Anglo-Australian Telescopeās 2dF/HERMES (High Efficiency and Resolution Multi-Element Spectrograph) system, based upon spectra acquired during the first two years of the Galactic Archaeology with HERMES survey. Averaged spectral fluxes of stars were compared to their photometry to determine the relative throughputs of fibres for a range of fibre position and atmospheric conditions. We find that overall the throughputs of the 771 usable fibres have been stable over the first two years of its operation. About 2.5 per cent of fibres have throughputs much lower than the average. There are also a number of yet unexplained variations between the HERMES bandpasses, and mechanically and optically linked fibre groups known as retractors or slitlets related to regions of the focal plane. These findings do not impact the science that HERMES will produce.
2015
First light results from the High Efficiency and Resolution Multi-Element Spectrograph at the Anglo-Australian Telescope
Andrew Sheinis, Borja Anguiano, Martin Asplund, Carlos Bacigalupo, Sam Barden, Michael Birchall, Joss Bland-Hawthorn, Jurek Brzeski, Russell Cannon, Daniela Carollo, Scott Case, Andrew Casey, Vladimir Churilov, Couch Warrick, Robert Dean, Gayandhi De Silva, Valentina DāOrazi, Ly Duong, Tony Farrell, Kristin Fiegert, Kenneth Freeman, Frost Gabriella, Luke Gers, Michael Goodwin, Doug Gray, Andrew Green, Ron Heald, Jeroen Heijmans, Michael Ireland, Damien Jones, Prajwal Kafle, Stefan Keller, Urs Klauser, Yuriy Kondrat, Janez Kos, Jon Lawrence, Steve Lee, Slavko Mali, Sarah Martell, Darren Mathews, Don Mayfield, Stan Miziarski, Rolf Muller, Naveen Pai, Robert Patterson, Ed Penny, David Orr, Katharine Schlesinger, Sanjib Sharma, Keith Shortridge, Jeffrey Simpson, Scott Smedley, Greg Smith, Darren Stafford, Nicholas Staszak, Minh Vuong, Lewis Waller, Elizabeth Wylie Boer, Pascal Xavier, Jessica Zheng, Ross Zhelem, Daniel Zucker, and Tomaz ZwitterJATIS, 2015The High Efficiency and Resolution Multi Element Spectrograph, HERMES, is a facility-class optical spectrograph for the Anglo-Australian Telescope (AAT). It is designed primarily for Galactic Archaeology, the first major attempt to create a detailed understanding of galaxy formation and evolution by studying the history of our own galaxy, the Milky Way. The goal of the GALAH survey is to reconstruct the mass assembly history of the Milky Way through a detailed chemical abundance study of one million stars. The spectrograph is based at the AAT and is fed by the existing 2dF robotic fiber positioning system. The spectrograph uses volume phase holographic gratings to achieve a spectral resolving power of 28,000 in standard mode and also provides a high-resolution mode ranging between 40,000 and 50,000 using a slit mask. The GALAH survey requires an SNR greater than 100 for a star brightness of V=14 in an exposure time of one hour. The total spectral coverage of the four channels is about 100 nm between 370 and 1000 nm for up to 392 simultaneous targets within the 2-degree field of view. HERMES has been commissioned over three runs, during bright time in October, November, and December 2013, in parallel with the beginning of the GALAH pilot survey, which started in November 2013. We present the first-light results from the commissioning run and the beginning of the GALAH survey, including performance results such as throughput and resolution, as well as instrument reliability.
The GALAH survey: scientific motivation
G. M. De Silva, K. C. Freeman, J. Bland-Hawthorn, S. Martell, E. Wylie Boer, M. Asplund, S. Keller, S. Sharma, D. B. Zucker, T. Zwitter, B. Anguiano, C. Bacigalupo, D. Bayliss, M. A. Beavis, M. Bergemann, S. Campbell, R. Cannon, D. Carollo, L. Casagrande, A. R. Casey, G. Da Costa, V. DāOrazi, A. Dotter, L. Duong, A. Heger, M. J. Ireland, P. R. Kafle, J. Kos, J. Lattanzio, G. F. Lewis, J. Lin, K. Lind, U. Munari, D. M. Nataf, S. OāToole, Q. Parker, W. Reid, K. J. Schlesinger, A. Sheinis, J. D. Simpson, D. Stello, Y. -S. Ting, G. Traven, F. Watson, R. Wittenmyer, D. Yong, and M. ŽerjalMNRAS, 2015The Galactic Archaeology with HERMES (GALAH) survey is a large high-resolution spectroscopic survey using the newly commissioned High Efficiency and Resolution Multi-Element Spectrograph (HERMES) on the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ā¼ 28 000) spectra in four passbands for 392 stars simultaneously over a 2 deg field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V ā¼ 14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, Ī±-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.
2013
Spectral matching for abundances of 848 stars of the giant branches of the globular cluster Ļ Centauri
Jeffrey D. Simpson, and P. L. CottrellMNRAS, 2013We present the effective temperatures, surface gravities and abundances of iron, carbon and barium of 848 giant branch stars, of which 557 also have well-defined nitrogen abundances, of the globular cluster Ļ Centauri. This work used photometric sources and lower resolution spectra for this abundance analysis. Spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen rich or oxygen poor.
Spectral matching for elemental abundances of evolved stars of globular clusters
Jeffrey D. Simpson2013
2012
Spectral matching for abundances and clustering analysis of stars on the giant branches of Ļ Centauri
Jeffrey D. Simpson, P. L. Cottrell, and C. C. WorleyMNRAS, 2012We have determined stellar parameters and abundances for 221 giant branch stars in the globular cluster Ļ Centauri. A combination of photometry and lower resolution spectroscopy was used to determine temperature, gravity, metallicity, [C/Fe], [N/Fe] and [Ba/Fe]. These abundances agree well with those found by previous researchers and expand the analysed sample of the cluster. k-means clustering analysis was used to group the stars into four homogeneous groups based upon these abundances.
These stars show the expected anticorrelation between [C/Fe] and [N/Fe]. We investigated the distribution of CN-weak/CN-strong stars on the colour-magnitude diagram. Asymptotic giant branch stars, which were selected from their position on the colour-magnitude diagram, were almost all CN weak. This is in contrast to the red giant branch where a large minority were CN strong. The results were also compared with cluster formation and evolution models. Overall, this study shows that statistically significant elemental and evolutionary conclusions can be obtained from lower resolution spectroscopy.