Publications
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(A click on the below tab will lead you to our published works!)
Lithium in red giants: the roles of the He-core flash and the luminosity bump. Deepak and Lambert D. L., 2021 MNRAS, 507, 205-224. doi: https://doi.org/10.1093/mnras/stab2022
Abstract: Lithium abundances for red giants in the GALAH DR3 survey are studied. The rare examples of Li-enriched stars with abundances A(Li) ≥1.5 are confirmed to be He-core burning stars belonging to or evolved from the red clump with similar masses and metallicity: M ≃ 1.1 ± 0.2 M⊙ and [Fe/H] ≃ − 0.3 ± 0.3. Li enrichment over the Li abundance present in a star's predecessor at the tip of the red giant branch likely occurs in all these red clump stars. Examination of the elemental abundances (C to Eu) in the GALAH catalogue shows no anomalous abundances in red clump giants and, in particular, no dependence on the Li abundance, which ranges over at least five dex. Lithium synthesis is attributed to the He-core flash occurring in stars at the tip of the red giant branch. Models from the Modules for Experiments in Stellar Astrophysics (MESA) match the observed evolution of these stars along the red giant branch and to the red clump but only at the low effective temperature end of the observed spread of red clump giants. Run of Li abundance on the red giant branch is fairly well reproduced by MESA models. A speculation is presented that the series of He-core flashes not only leads to 7Li synthesis from a star's internal reservoir of 3He but also may lead to internal restructuring leading to the observed effective temperature spread of red clump stars at about a constant luminosity. Giants exhibiting marked Li enrichments are not found at other evolutionary phases and, in particular, not directly associated with the luminosity bump on the red giant branch for which the Li abundance increase does not exceed 0.3 dex.
Data available on Zenodo at https://doi.org/10.5281/zenodo.5086893
Lithium abundances and asteroseismology of red giants: understanding the evolution of lithium in giants based on asteroseismic parameters. Deepak and Lambert D. L., 2021, MNRAS, 505, 642. doi: 10.1093/mnras/stab1195
Abstract: In this study, we explore the evolution of lithium in giant stars based on data assembled from the literature on asteroseismology and Li abundances for giants. Our final sample of 187 giants consists of 44 red giant branch (RGB), 140 core He-burning (CHeB) and three giants with an unclassified evolutionary phase. For all 187 stars, the seismic parameters νmax (frequency of maximum oscillation power) and Δν (large frequency spacing) are available, while ΔΠ1 (the asymptotic gravity-mode period spacing) is available for a subset of 64. For some of the CHeB giants, mass estimates from the asteroseismic scaling relations are found to be underestimated when compared with mass estimates from isochrones based on seismic data. Whilst most of the Li-rich giants in the sample have masses less than 1.5 M⊙, they are also present up to and beyond the maximum mass expected to have suffered a core He-flash, i.e. M ≤ 2.25 M⊙: this suggests contributions from other processes towards Li enrichment. To understand the evolution of giants in the ΔΠ1 - Δν plane, we use the Modules for Experiments in Stellar Astrophysics models which show the presence of mini He-flashes following the initial strong core He-flash. From the distribution of A(Li) as a function of Δν, which is similar to the distribution of A(Li) as a function of luminosity, we find no indication of Li enrichment near the luminosity bump. Also, A(Li) trends to ∼ -1.5 dex near the RGB tip. The data also suggest a decrease in A(Li) with an increase in ΔΠ1 for CHeB giants.
Data available on Zenodo at https://doi.org/10.5281/zenodo.4519625
Abstract: Compositions of lithium-enriched and normal giants among the GALAH survey are compared. Except for Li, the only detectable abundance difference between lithium-enriched and normal giants among the investigated elements from carbon to europium occurs for carbon. Among Li-rich giants with A(Li) = 1.8–3.1, 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 underabundant 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 12C/13C ratios from the literature suggests that the addition of Li to create a Li-rich giant may occur independently of the abundance changes wrought by the first dredge-up. The 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 the occurrence of Li-enriched giants among normal giants is about 1 per cent and slightly dependent on metallicity ([Fe/H]). Li-enriched and normal giants are found to have similar projected rotational velocity, which suggests that Li enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars.
Abstract: We discuss the evolution of Li in the Galaxy based on a large sample of dwarfs and giants collected from the cross-match of the Gaia and GALAH surveys. Data spans a wide range in metallicity and cover various Galactic evolutionary phases. Metal-poor dwarfs with [Fe/H] < -0.8 dex are found to have constant mean A(Li) = 2.2 dex with a standard deviation of 0.14 dex but with a very slightly increasing trend with an increase in metallicity. However, in the sub-solar metallicity range (-0.8 < [Fe/H] < 0 dex), Li has increased significantly from the initial value of about 2.2 dex to the present value of about 3.3 dex. However, in super-solar metallicity stars, Li is found to be decreasing with increasing metallicity. We also discuss briefly about a rare class of so-called Li-rich low-mass giants as one of the promising candidates for Li enrichment in the Galaxy.
Abstract: Here, we explore the enrichment of Lithium in the Galaxy using a large sample of stars common among large spectroscopic surveys such as the GALAH and astrometric survey by the Gaia satellite. For this study we used about 60,000 low mass (M⩽ 2M⊙) dwarfs from the GALAH survey. Further, we discuss Li enrichment among giant stars based on a sample of 52,000 low mass giants, of which 335 are Li-rich with A(Li) ⩾ 1.80 ± 0.14 dex, culled from the GALAH survey. These low mass giants appears to be one of the promising source of Li enrichment in the Galaxy as their atmospheric Li can be added to the ISM through mass loss.
Abstract: In this article, we speculate on the possible mechanisms for Li enhancement origin in RGB stars based on a large data set of around 340 299 stars collected from the GALAH survey combined with the Gaia astrometry. Data has 51 982 low mass (M ≤ 2 M⊙) RGB stars with reliable atmospheric parameters. The data set shows a well-populated RGB with a well-defined luminosity bump and a red clump with significant number of stars at each of these two key phases. We found 335 new Li-rich RGB stars with Li abundance, A(Li) ≥ 1.80 ± 0.14 dex, of which 20 are super Li-rich with A(Li) ≥ 3.20 dex. Most of them appear to be in the red clump region which, when combined with stellar evolutionary time scales on RGB, indicates that the Li enhancement origin may lie at RGB tip during He-flash rather than by external source of merging of sub-stellar objects or during luminosity bump evolution. Kinematic properties of sample stars suggest that Li-rich giants are relatively more prevalent among giants of thin disc compared to thick disc and halo.
Abstract: Here, we report a comparative study of radial velocity (RV) data of two major surveys: Gaia Data Release 2 and RAVE Data Release 5. We restricted the sample to stars with relatively accurate RVs (σRV_Gaia ≤ 2 km s−1 or ≤ 2%, and σRV_Rave ≤ 2 km s−1 or ≤ 2%). The difference between RVGaia and RVRAVE for a majority of the sample follows a normal distribution with mean = 0.28 km s−1 and σ = 1.49 km s−1. However, we found a very small group of stars (≈ 0.08% of the total) for which the difference in RVs between the two surveys is significantly larger with an offset of −104.50 km s−1 with σ = 4.92 km s−1. Kinematics based on RVGaia suggest that most of the group members belong to the Galactic thin disk, which agrees with the group's metallicity range of −1.2 to +0.5 dex suggesting the offset in RV is probably due to RAVE velocity data for this particular group.