Study of lithium and other related elements among evolved stars

Abstract

The formation and evolution of elements is a fundamental problem in modern astrophysics. Lithium (Li), one of the four stable nuclides produced during Big Bang Nucleosynthesis (BBN), exhibits an observed abundance in many celestial bodies that deviates from theoretical predictions. In stars, the behavior of Li is particularly complex, as it is easily destroyed under high temperatures, where it undergoes proton capture to form stable helium nuclei. Consequently, Li cannot be preserved in the hot stellar interior, but is instead confined to the surface layers. If surface Li is transported to the interior during stellar evolution it is destroyed due to high temperatures, leading to a reduction in Li abundance in the star’s atmosphere. Standard stellar evolution models predict that stars with initial Li abundances comparable to the ISM value ( 3.3 dex) will show a Li abundance no greater than 1.5 dex once they reach the giant stage. However, observations have revealed a small number of giant stars, known as Li-rich giants, with Li abundances exceeding this threshold. Some of these stars even display Li abundances higher than the ISM value, and are referred to as super Li-rich giants. The presence of such stars presents a significant challenge to the standard stellar evolution models, suggesting the existence of additional mechanisms that can enrich Li in stars. Understanding these mechanisms is essential for both the formation of Li and the broader theories of stellar evolution, though it remains a challenging area of research. Significant progress has been made over the past four decades in the study of Li-rich giants. We have used large scale survey programs such as LAMOST and GALAH spectroscopic surveys, photometric survey of Kepler and TESS space telescopes, Gaia astrometry and high-resolution spectra using 2-m Himalayan Chandra Telescope (HCT) and the 10 m Hobby Eberly Telescope (HET). Our work discovered a number of new red clump super-Li rich giants with proper evolutionary phases assigned based on asteroseismic analysis. The extensive systematic survey based on an unbiased sample of giants common among Kepler and LAMOST surveys’ fields provided a key evidence that Li enhancement is only associated with the He-core burning phase post He-flash in low-mass stars. Another key result that emerged from this study is the location of the Li enrichment site in most Li-rich stars is the He-flashing phase, the transition between the evolution of stars from the end of RGB tip to the He-core burning period in the horizontal branch. Based on the analysis of spectroscopic and photometric data, we provided first-of-its-kind evidence in the form of a correlation between lithium abundances in giants and their masses derived using asteroseismology. Another significant part of this thesis is the study of photospheric Li abundances and its relation with the strength of chromospheric He I 10830Å. The subtle correlation is the another evidence that the He-flash is the source for Li enhancement and the enhanced activity in the chromosphere resulting stronger He I line among very high Li-rich giants.