Three-dimensional non-local thermodynamic equilibrium radiative transfer and oxygen abundances in late-type stars

Abstract

The chemical compositions of the atmospheres of late-type stars, as inferred from stellar spectroscopic analyses, provide vital clues to unravelling the history of stars, galaxies, and the cosmos as a whole. However, the vast majority of stellar spectroscopic analyses make at least two assumptions that severely limit their accuracy: that stellar atmospheres are one-dimensional (1D) and hydrostatic; and that the material in the line-forming regions is in local thermodynamic equilibrium (LTE). Real atmospheres of late-type stars have convective envelopes that require a 3D time-evolving hydrodynamical treatment, and also real atmospheres are generally not in LTE. In this thesis I develop tools for 3D non-LTE radiative transfer calculations in late-type stars, and use them to address two outstanding problems that are pertinent to oxygen, which is one of the most important elements in astronomy. First is the so-called solar mod- elling problem, wherein inferences about the structure of the Sun based on helioseismology are in significant disagreement with those inferences based on the current best estimate of the solar chemical composition (as deduced from spectroscopy) and standard solar interior models. It has been strongly argued in the literature that a higher solar oxygen abundance is needed to resolve this problem. Second is the so-called oxygen problem in metal-poor stars, wherein different oxygen abundance diagnostics give different oxygen abundances in metal-poor Milky Way disk and halo stars. In particular, this has meant that the Galactic [O/Fe] versus [Fe/H] trend, a key tracer of chemical evolution, is poorly constrained in the metal-poor regime. I present new 3D non-LTE analyses of oxygen and silicon lines in the solar spectrum. The inferred solar oxygen and silicon abundances, 8.70 ± 0.03 dex and 7.51 ± 0.03 dex respectively, are consistent with the current canonical values to within errors, so maintaining the status quo on the solar modelling problem. I also present 3D non-LTE spectra for atomic oxygen lines across a grid of 3D hydrodynamic model atmospheres. Such a grid facilitates 3D non-LTE analyses of stars other than the Sun. With this grid I present analyses of the [O/Fe] versus [Fe/H] trend from Galactic disk and halo stars, and I demonstrate that with accurate stellar parameters and 3D non-LTE modelling, concordant results can be achieved between the two key atomic oxygen diagnostics: the [Oi]630nm line, and the O i 777 nm lines. Lastly, I present a 3D non-LTE analysis of Fe i and Fe ii lines in four metal- poor benchmark stars: HD84937, HD122563, HD140283, and G64-12. I demonstrate that the 3D non-LTE iron abundances are typically 0.1 dex higher than the corresponding 1D non-LTE iron abundances. 3D effects of this order need to be accounted for if the Galactic [O/Fe] versus [Fe/H] relationship is to be properly constrained.