The influence of convection on the atmospheric structures and observable properties of red giant stars

TL;DR

This paper reviews how 3D hydrodynamical models reveal significant differences in the atmospheric structures and spectral line predictions of red giant stars compared to traditional 1D models, especially at low metallicities.

Contribution

It provides an overview of recent advances in 3D hydrodynamical modeling of red giant atmospheres and highlights discrepancies with 1D models, emphasizing the importance of 3D effects.

Findings

3D models show significant atmospheric differences from 1D models.

Large temperature fluctuations affect spectral line strengths.

Discrepancies in abundance measurements can reach -0.6 dex at low metallicity.

Abstract

During the recent years significant progress has been made in the modeling of red giant atmospheres with the aid of 3D hydrodynamical model atmosphere codes. In this contribution we provide an overview of selected results obtained in this context by utilizing 3D hydrodynamical CO5BOLD stellar model atmospheres. Hydrodynamical simulations show that convective motions lead to significant differences in the atmospheric structures of red giants with respect to those predicted by the classical 1D model atmospheres. Results of these simulations also show that in certain cases 1D models fail to reproduce even the average properties of the 3D hydrodynamical models, such as P-T profiles. Large horizontal temperature fluctuations in the 3D model atmospheres, as well as differences between the temperature profiles of the average <3D> and 1D models, lead to large discrepancies in the strengths of spectral lines predicted by the 3D and 1D model atmospheres. This is especially important in models at lowest metallicities ([M/H]<-2.0) where the 3D-1D abundance differences may reach (or even exceed) -0.6 dex for lines of neutral atoms and molecules. We also discuss several simplifications and numerical aspects involved in the present 3D hydrodynamical modeling of red giant atmospheres, and briefly address several issues where urgent progress may be needed.