Name: Anohita Mallick
Affiliation: Indian Institute Of Astrophysics
Conference ID : ASI2022_278
Title : Probing Li enhancement­-IR excess connection in stars close to RGB tip
Authors : Anohita Mallick, Bacham E. Reddy, C. Muthumariappan
Abstract Type: Poster
Abstract Category : Stars, ISM and Galaxy
Abstract : As stars leave subgiant branch, lithium abundance decreases because Li produced in surface layers is burnt by convective dilution. But there are giants with large Li abundance and in some of these it is even larger than ISM (A(Li) = 3.2) – which indicates that Li enrichment must have occurred in them. After core hydrogen fusion in Main sequence, stellar envelope in RGBs becomes fully convective. At RGB tip central temperature and density are high enough for quantum tunneling to overcome Coulomb barrier between the nuclei, allowing triple­ alpha process to begin. As core is now degenerate, it does not expand and Helium consumption occurs very rapidly known as “Helium flash”. Core degeneracy is lifted and He core burning initiates in these “Red Clump Stars”. Recent works demonstrated A(LI) increases by a factor of ∼40 from RGB tip to RC. Li enhancement must occur between TRGB and RC. There are two possibilities – enhancement by merger with other stellar objects or internal nucleosynthesis in RC stars. For the latter some “extra mixing mechanism” is responsible for connecting outer convective envelope and inner H burning shell. For the first possibility these might be - 1) engulfment of close­in planet(s) during RGB phase 2) White dwarf + red giant mergers. It's still not understood which of these mechanisms is responsible for anomalous lithium abundances close to TRGB. We investigated a large sample of low mass red giants on upper RGB and RC phase from GALAH DR3 survey. IR excess was found in some using infrared photometry implying high A(Li) in atleast some of these giants is due to mergers. We utilized DUSTY code for IR excess sources to estimate dust parameters. WISE band observations were used to constrain geometrical distribution of dust shell. Additionally, rotational velocities were considered to understand mass­-loss mechanism.