Understanding of Li production and its evolution in evolved stars

Abstract

Lithium (Li) is susceptible to relatively low temperatures. Hence, Li abundances among red giants are expected to be depleted signi cantly, by a factor of 40-60, from its original value on the main sequence. Assuming A(Li)= 3.2 dex on the main sequence, it is predicted maximum of A(Li)=1.6 - 1.8 dex, depending on mass, for RGB giants. In literature, giants with A(Li) 1.6 dex, in general, termed as Li-rich. The Li excess anomaly has been a puzzle to stellar evolutionary models ever since its serendipitous discovery in the early 1980s. This thesis aims to understand the anomaly among a small group of low mass red giant branch (RGB) stars. Understanding of the Li excess phenomenon in stars has broader implications for our better understanding of Li evolution in the universe and in stars, which in turn would help to better constrain theoretical models of stellar evolution. We used large scale surveys for data collection such as LAMOST spectroscopic survey, photometric survey of Kepler space telescope, Gaia astrometry and high-resolution spectra using 2-m Himalayan Chandra Telescope (HCT). Our work discovered several red clump super-Li rich giants for the rst time with an unambiguous evolutionary phase based on asteroseismic analysis. The extensive systematic survey based on an unbiased sample of giants common among Kepler and LAMOST surveys' elds provided a key evidence that Li enhancement is only associated with the He-core burning phase post- He-ash. Another key result that emerged from this study is the location of the Li enrichment site during the He- ashing phase, the transition phase between the evolution of stars from the end of RGB tip to the He-core burning phase of the red clump phase. Based on the analysis of spectroscopic and photometric data, we provided rst-of-its-kind evidence in the form of a correlation between lithium abundances in giants and period spacing of g-mode oscillations derived using asteroseismology. The evidence being that all the super Li-rich giants are almost exclusively young red clump giants compared to Li-poor red clump giants suggesting the direct connection between the He- ash occurrence and the presence of Li in red clump giants. We have set robust upper limits of 40 million years, since the on-set of He- ash, within which period Li enhancement occurred. However, data does indicate Li enhancement occurred much earlier, probably, during the period of He- ashing phase itself i.e within two million years from the on-set of He- ash at the tip of RGB. The later suggestion requires more re ned data of a larger sample.