Abundance analyses of Li-enriched and normal giants in the GALAH survey

Abstract

Compositions of lithium-enriched and normal giants among the GALAH survey are compared. Except for Li, the only detectable abundance difference between lithiumenriched and normal giants among the investigated elements from carbon to europium occurs for carbon. Among Li-rich giants with A(Li) = 1.8 to 3.1, the C deficiency is very similar to that reported for the normal giants (with A(Li) < 1.8) where the slight C deficiency arises from the first dredge-up. Carbon is slightly under abundant relative to normal giants among the super Li-rich giants where the Li abundance exceeds A(Li) = 3.2. The C abundance as well as the 12 C/ 13 C ratios from the literature suggest that addition of Li to create a Li-rich giant may occur independently of the abundance changes wrought by the first dredge-up. Creation of a super Li-rich giant, however, appears to occur with additional CN-cycle conversion of C to N. The probability of becoming a Li-rich giant is approximately independent of a star's mass, although the majority of the Li-rich giants are found to be low mass (M ≤ 2 M ). The frequency of occurrence of Li-enriched giants among normal giants is about one percent and slightly dependent on metallicity ([Fe/H]). Li-enriched and normal giants are found to have similar projected rotational velocity which suggest that Li-enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars.

The GALAH survey: A census of lithium-rich giant stars

“…Recent works with larger data sets (e.g. Casey et al 2019;Deepak et al 2020) have quantified this as a more general increase in the occurrence rate of lithium-rich giants with increasing metallicity. In Fig.…”

“…There are only a few elements in GALAH DR3 that show any obvious differences between the abundance patterns of Li-normal and Li-rich giants. In this section, we discuss [C/Fe] , as it was recently highlighted by Deepak et al (2020) as showing possible correlations with lithium enrichment. Appendix A discusses an apparent correlation between the occurrence rate of lithium-rich RC stars and α enhancement that is driven by the metallicity dependence discussed in Section 3.1.…”

“…Appendix A discusses an apparent correlation between the occurrence rate of lithium-rich RC stars and α enhancement that is driven by the metallicity dependence discussed in Section 3.1. Deepak et al (2020) recently explored the other elemental abundances of Li-rich and Li-normal giants using the GALAH DR2 data set. They found that for all the elements available, the only element that showed an appreciable difference between the two populations was carbon.…”

“…In this study, we follow the lead of Casey et al (2019) and Deepak et al (2020), and look at the probability for a star in a given evolutionary phase to be lithium-rich, rather than the probability for a lithium-rich star to be in a given evolutionary phase. The overall occurrence rate is known to lie around 1 per cent, but subdividing the data set into RC versus RGB stars and looking at the occurrence rate as a function of metallicity brings out some interesting behaviours.…”

“…With the availability of lithium abundances from large spectroscopic projects like the Gaia -ESO Survey (Gilmore et al 2012), the LAMOST survey (Cui et al 2012), and the GALAH Survey (De Silva et al 2015), and the tremendous expansion in asteroseismic sky coverage from the Kepler (Stello et al 2013), K2 (Stello et al 2017), and TESS (Silva Aguirre et al 2020) missions, we can now assemble and use large catalogues of lithium-rich giants with reliably determined evolutionary states. Recent works (Casey et al 2019;Deepak & Reddy 2019;Gao et al 2019;Singh et al 2019b;Deepak, Lambert & Reddy 2020) have identified thousands of lithium-rich giants in the Milky Way, a major expansion from the previous small samples.…”