Tomography of cool giant and supergiant star atmospheres. I. Validation of the method

TL;DR

This paper validates a tomographic method to analyze velocity fields in cool giant and supergiant star atmospheres, using 1D models and 3D simulations to accurately recover spectral line formation depths and atmospheric dynamics.

Contribution

It introduces and validates a tomographic technique for probing velocity fields at different depths in stellar atmospheres, incorporating contribution functions for spectral line formation.

Findings

The method accurately recovers velocity distributions in 3D simulations.

Spectral line formation depths differ significantly between 1D and 3D models.

The technique effectively distinguishes atmospheric inhomogeneities in 3D simulations.

Abstract

Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. The tomographic method is applied to 1D model atmospheres and to a realistic 3D radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of CCF profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.