Chemodynamic studies of the galaxy

Abstract

Based on data from large spectroscopic surveys like the GALactic Archaeology with HERMES (GALAH) and the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), along with astrometric and photometric data from the Gaia survey, we have addressed three major questions in the field of the chemical and dynamical evolution of the Galaxy. The first of these three questions is understanding the evolution of lithium (Li) in the Galaxy. For this, we used the spectroscopic data from the GALAH survey along with astrometric and photometric data from the Gaia survey. We found that Li has significantly increased in the Galactic disc compared to the observed value in the metal-poor halo stars and the theoretically predicted primordial value from the Standard Big Bang Nucleosynthesis models. Search for an explanation for such large-scale Li enrichment throughout the Galactic disc led us to study the evolution of Li in low-mass stars (the second problem we have addressed). According to the standard evolutionary models, Li gets depleted as stars ascend on the red-giant branch (RGB). However, about a per cent of giants have been found to have a thousandfold higher Li than the value predicted by standard models. These rare giants with high Li, well known as the Li-rich giants, have puzzled astronomers for over four decades since their first discovery by Wallerstein and Sneden (1982). To understand the evolution of Li in low-mass stars, we used data from the GALAH and Gaia surveys. From the second data release of the GALAH survey, we discovered 335 new Li-rich giants (with A(Li) ≥ 1.8 dex), of which 20 are super Li-rich with A(Li) ≥ 3.2 dex. We further discovered that almost all of these Li-rich giants are core He-burning (CHeB) giant stars, and Li enrichment in red giants is likely associated with the He-core flash. We also found that the Li-rich giants do not belong to any particular dynamic group in the Galaxy and are found throughout the Galaxy. However, they are more prevalent among giants of the Galactic thin disc than the thick disc and halo. We further found that chemically Li-rich and normal giants are similar except for Li abundance. We also found that 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 occurrence of Lienriched giants among normal giants is about one per cent and slightly dependent on metallicity. Li-enriched and normal giants are also found to have a similar projected rotational velocity, suggesting that Li-enrichment in giants is not linked to scenarios such as mergers and tidal interaction between binary stars. To find more clues about the origin of Li enrichment in giants, we studied the correlation between giants’ Li abundance and asteroseismic parameters. Data for the CHeB giants suggest a decrease in A(Li) with an increase in the gravity mode period spacing ∆Π1, which is known to increase with time at the start of the CHeB phase suggesting the enriched Li in He-core burning giants decreases as stars evolve. Based on asteroseismic data also, we found no evidence of Li enrichment at the luminosity bump. In conclusion, these studies have helped to uncover the almost four-decade-old mystery of the origin of Li-rich giant stars, and now we know that all the low-mass giants experience Li enrichment during the CHeB phase to a varying degree. Lastly, to uncover the formation and evolution history of the Galaxy, we studied the chemical and kinematic properties of various pro-grade (like the Splash) and retrograde (like Gaia-Enceladus/Sausage (GE/S), Thamnos and Sequoia) substructures in the Galactic halo along with the Galactic thin and thick disc. We also studied the age distributions of all these Galactic components. We found that the star formation in the Splash, which is the major in situ component of the halo and has a median [Fe/H] of −0.75±0.24 dex, peaked about 13 Gyr ago. On the other hand, the star formation in the GE/S, which is the halo’s largest accreted component and has a median [Fe/H] of −1.31 ± 0.23 dex, peaked about 1.5 Gyr later than the Splash. The Galactic thin and thick discs are found to have peak ages of about 5.0 and 11.5 Gyr, respectively. The GE/S and Sequoia are also found to have a distinct chemical evolution from the Splash, whose chemical composition is found to be similar to the metal-poor stars of the thick disc. In conclusion, our studies support the idea of galaxy formation by hierarchical clustering in a Lambda cold dark matter universe.