Low-mass Stellar Evolution Traced with Non-LTE Abundances

Abstract

The detailed chemical composition of stellar atmospheres can reveal the structure and evolution of the stellar interiors, otherwise hidden from direct site, as well as the structure and evolution of our entire Galaxy. The advent of several large-scale stellar spectroscopic surveys promises breakthroughs in our understanding of the physical processes that shape stellar surface abundances. However, the full potential of these extremely large and precise surveys is not yet being reached, as standard elemental abundance determinations today are based on the simplifying and incorrect assumption that the stellar atmosphere is in local thermodynamic equilibrium (LTE). In this thesis I have employed non-LTE radiative transfer methods to tackle two outstanding astrophysical problems. The first problem is related to the chemical homogeneity in the open clusters, which for example is very important to understand how disrupted clusters have formed the Galactic disk and pinpoint the birth location of field stars. Abundance trends with stellar effective temperature have been found in all the analysed elements, indicating that the chemical abundance varies along with evolutionary phase past the turn-off. The overall agreement between our measured abundance patterns and the predictions by the stellar models with atomic diffusion and mixing, implies that the process of atomic diffusion poses a non-negligible effects during the main-sequence phase, which leads to the inhomogeneities in the abundances of open clusters. The second problem is related to lithium evolution in low-mass main-sequence stars. The primordial elemental abundances predicted by Standard Big Bang nucleosynthesis (SBBN) generally show good agreement with observations. However, a glaring exception is the cosmic abundance of lithium, which SBBN estimates to be three times higher than what is observed in the atmospheres of metal-poor stars in the Galactic halo (i.e. stars on the so-called Spite Plateau). This long-recognized discrepancy has become known as the Cosmological Lithium Problem. In this thesis, I present observational evidence, based on a state-of-the-art non-LTE spectroscopic analysis of more than 100,000 stars from the large-scale spectroscopic “Galactic Archaeology with HERMES(GALAH) survey, that the surface lithium abundances of these Spite Plateau do not in fact reflect their initial (SBBN) lithium abundances; rather, they have been depleted by a factor of three. This further strengthens the case for an astrophysical solution to the cosmological problem, reconciling tension with predictions of the SBBN.