Stellar Atmospheres

TL;DR

This paper reviews the methods for modeling stellar atmospheres, emphasizing the physics involved, including radiative transfer, convection, and magnetic effects, to interpret stellar spectra and understand star properties.

Contribution

It provides a comprehensive overview of the theoretical foundations and computational techniques for stellar atmosphere modeling, including LTE assumptions and non-LTE effects.

Findings

Highlights the importance of opacities and source functions in models.

Discusses the impact of stellar winds, rotation, and magnetic fields.

Outlines procedures for quantitative spectral analysis.

Abstract

Stars play a decisive role in our Universe, from its beginning throughout its complete evolution. For a thorough understanding of their properties, evolution, and physics of their outer envelopes, stellar spectra need to be analyzed by comparison with numerical models of their atmospheres. We discuss the foundations of how to calculate such models (in particular, density and temperature stratification, affected by convective energy transport in low-mass stars), which requires a parallel treatment of hydrodynamics, thermodynamics and radiative transfer. We stress the impact of emissivities, opacities, and particularly their ratio (source function), and summarize how these quantities are calculated, either adopting or relaxing the assumption of LTE (local thermodynamic equilibrium). Subsequently, we discuss the influence and physics of stellar winds (and their various driving mechanisms as a function of stellar type), rotation, magnetic fields, inhomogeneities, and multiplicity. Finally, we outline the basics of quantitative spectroscopy, namely how to analyze observed spectra in practice.