Photon scattering 3D radiation-hydrodynamical simulations of late-type stellar atmospheres

Abstract

Numerical models of stellar atmospheres fulfill an important role in astrophysical research. They describe the physical environment from which stellar radiation originates, allowing detailed analyses of the temperature and pressure stratification in the atmosphere, of the chemical composition, atmospheric motion, phenomena related to magnetic fields and more. Theoretical studies of stellar atmospheres usually involve two steps: first, the model atmosphere is constructed according to a set of parameters. Based on this model, a theoretical representation of an observable is produced, e.g., spectral lines or the center-to-limb variation of continuum radiation. 3D time-dependent radiation-hydrodynamical simulations of stellar atmospheres have been very successful in reproducing observations and have become a viable tool for solar physics and stellar astrophysics. Coupling detailed radiative transfer with a hydrodynamical description of stellar surface granulation, they allow accurate predictions of the atmospheric stratification, velocity fields, spectral line formation, magnetic phenomena etc. The complexity and computational effort of 3D radiation-hydrodynamical simulations requires a variety of approximations, such as the assumption of local thermodynamic equilibrium (LTE) in the treatment of radiative transfer. This PhD thesis explores the enhancement of the radiation model with coherent photon scattering, studying its importance for radiative heating in 3D radiation-hydrodynamical simulations. By comparing the atmospheric temperature stratification derived from different treatments of scattering opacity, it is demonstrated that a solar-type photosphere is well-approximated when continuum scattering is treated as absorption, while this approach leads to significantly higher temperatures above the photosphere if applied to line-blanketing. In metal-poor giants, Rayleigh scattering is an important continuous opacity source; treating the opacity as absorption leads to significantly higher temperatures above the surface and a shallower temperature gradient. The temperature structure of the model atmosphere with coherent scattering can be approximated with reasonable accuracy by removing scattering opacity above the stellar surface and using a Planck source function, which largely reduces the computational effort. 3D spectral line formation is an essential diagnostic for simulations of stellar atmospheres, and a widely used tool for analyzing, e.g., the chemical composition of stars. Metal-poor giants are interesting astrophysical laboratories in this context, for studying the chemical evolution of the Galaxy and the origin of the elements. The second part of the thesis investigates LTE spectral line formation with continuum scattering in metal-poor giants. It is shown that an increasing thermalization depth through scattering at short wavelengths affects profile shapes and equivalent widths, with important consequences for measured abundances.