Jeffrey Simpson

selected publications

1. 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 D’Orazi, 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 Čotar, Thor Tepper-García, Jonathan Horner, Thomas Nordlander, Yuan-Sen Ting, Rosemary F. G. Wyse, and GALAH Collaboration

   MNRAS, 2021

   Abs arXiv HTML

   The 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 ’Spite plateau’ - 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 ’cosmological lithium problem’ - 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.

2. 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 Čotar, Jonathan Horner, Benjamin Montet, and Rob Wittenmyer

   MNRAS, 2021

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   We 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.

3. 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 Silva

   Natur, 2020

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   Globular 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 structure^2,3. Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a ‘metallicity floor’, whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun^4-6. The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form—both 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.

4. The closest extremely low-mass white dwarf to the Sun

   Adela Kawka, Jeffrey D. Simpson, Stéphane Vennes, Michael S. Bessell, Gary S. Da Costa, Anna F. Marino, and Simon J. Murphy

   MNRAS, 2020

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   We present the orbit and properties of 2MASS J050051.85-093054.9, establishing it as the closest (d ≍ 71 pc) extremely low-mass white dwarf to the Sun. We find that this star is hydrogen rich with T_\textrm eff≈10 500 K, log g ≍ 5.9, and, following evolutionary models, has a mass of ≍0.17 M_⊙. Independent analysis of radial velocity and Transiting Exoplanet Survey Satellite(TESS) photometric time series reveals an orbital period of ≍9.5 h. Its high velocity amplitude ( K≈144 \textrm km \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.

5. The GALAH Survey: Chemically tagging the Fimbulthul stream to the globular cluster ω 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 Čotar, Peter L. Cottrell, and Thomas Nordlander

   MNRAS, 2020

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   Using 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 ω 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 ω 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 ω Cen, which is rare in field stars. Apart from one star within 1.5 deg of ω Cen, we find no other stars observed by GALAH spatially near ω Cen or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to ω Cen confirms the earlier work, and further links this tidal feature in the Milky Way halo to ω Cen.

6. 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 Collaboration

   MNRAS, 2020

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   We 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 ∼2.35 M_⊙ A-type star located at a distance of ∼9 kpc from the Sun and has a heliocentric radial velocity of 1017 ± 2.7 km s^-1 without any signature of velocity variability. The current 3D velocity of the star in the Galactic frame is 1755 ± 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 ∼1800 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 V_y,⊙ = 246.1 ± 5.3 km s^-1 or position R₀ = 8.12 ± 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.

7. A nitrogen-enhanced metal-poor star discovered in the globular cluster ESO280-SC06

   Jeffrey D. Simpson, and Sarah L. Martell

   MNRAS, 2019

   Abs arXiv HTML

   We 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 ± 0.5 kpc, and find no significant change to its orbital properties.