| Abstract : | Aim of this thesis is to understand Li-excess anomaly among a small group of low mass red giant branch stars. Li is susceptible to relatively low temperature of 2.5MK. As a result, Li abundances are expected to be depleted 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. The anomaly, A(Li)>1.6, has been a puzzle to stellar evolutionary models ever since its serendipitous discovery in the early 1980s. There are several hypotheses for Li-excess in red giants. Some of them include planet engulfment, binary mergers, mass transfer from AGB, internal nucleosynthesis either at luminosity bump or RGB tip. One of the hindrances to constrain models is the lack of Li-rich giants' exact evolutionary phase. The thesis's primary goal is to provide unambiguous evolutionary phase of Li-rich giants and provide robust observational evidence of the site of Li-excess origin. We used large programmes such as LAMOST spectroscopic survey, Kepler photometric survey, Gaia astrometry, high-resolution spectra using 2-m HCT. Our suspicion that there may be only one unique process at a particular evolutionary phase of stars is responsible for Li-excess in giants was strengthened by our discovery of four new super Li-rich giants, for which evolutionary phase was determined based on direct asteroseismic and secondary calibration, [C/N], and 12C/13C. Our initial results prompted us to undertake a large systematic survey of giants common among Kepler and LAMOST fields. This work resulted in discovering new super-Li rich giants of red clump region based on asteroseismic analysis and the evidence that Li enhancement is only associated with the He-core burning phase post-He-flash at the RGB tip. |
