the red clump lithium problem

a project with a background image

Lithium is a crucial tool for investigating many processes in modern astrophysics. It is easily destroyed, and standard models of stellar evolution predict a clear decline in lithium abundance as stars evolve off the main sequence. Yet many studies have found that about one per cent of giant stars have large lithium abundances.

In we identified about 1000 lithium-rich giants in GALAH, and looked to understand their phenomenological properties. Combining GALAH with asteroseismic results, we found that such stars are twice as likely to be to be core-helium burning (red clump) than stars with low lithium abundances. The probability of being lithium-rich is also positively correlated with metallicity. Parallel to our research, made the provocative claim that there is in fact ubiquitous lithium production for all red clump stars. This would be a fundamental aspect of stellar evolution that has remained unrecognized until now. GALAH is perfect observational dataset to combine with stellar evolution models to explain this phenomenon.

I have found that this claim from is very likely true. By contrasting GALAH data against a Galaxia mock catalogue of the Milky Way , I find that there is a significant excess of red clump stars observed by GALAH that have measurable lithium abundance. This is demonstrated in Figure 1, which compares the distribution of stars with measurable lithium abundances from GALAH to a Galaxia mock catalogue. For the Galaxia stars, we are able to turn off and on the lithium abundance for the red clump stars — stars that are predicted to have negligible lithium abundances by the standard model of lithium evolution. For GALAH, we find that about 10 per cent of giant stars have lithium measurements, with no obvious deficit at the red clump luminoisity; while for the Galaxia sample, there is a clear dip at the red clump luminosity when we have turned off the lithium.

This raises some questions: Where are the red clump stars getting lithium from? Are lithium-rich red clump stars simply the outliers in a universal lithium enrichment process? There are several proposed explanations . I have been working with a student to develop an empirical model of lithium evolution from the helium flash through the red clump phase, and I am developing that into my next first-author publication.