3D simulations of M star atmosphere velocities and their influence on molecular FeH lines

TL;DR

This study uses 3D hydrodynamical simulations to analyze velocity fields in M star atmospheres and their impact on FeH molecular line shapes, providing insights into when 3D modeling is essential.

Contribution

It characterizes the dependence of velocity fields on Teff and log g in M stars and compares 3D velocity fields with 1D turbulent velocities for spectral synthesis.

Findings

Velocity fields increase with higher Teff and lower log g.

FeH line shapes are significantly affected by velocity fields.

3D effects are crucial for accurate spectral modeling in certain parameter ranges.

Abstract

We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on FeH molecular line shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff of 2500 K and 4000 K. Our aim is to characterize the Teff- and log g -dependence of the velocity fields and express them in terms of micro- and macro-turbulent velocities in the one dimensional sense. We present also a direct comparison between 3D hydrodynamical velocity fields and 1D turbulent velocities. The velocity fields strongly affect the line shapes of FeH, and it is our goal to give a rough estimate for the log g and Teff parameter range in which 3D spectral synthesis is necessary and where 1D synthesis suffices. In order to calculate M-star structure models we employ the 3D radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these models is performed with the line synthesis code LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian standard deviation and project them onto the line of sight to include geometrical and limb-darkening effects. The micro- and macro-turbulent velocities are determined with the "Curve of Growth" method and convolution with a Gaussian velocity profile, respectively. To characterize the log g and Teff dependence of FeH lines, the equivalent width, line width, and line depth are regarded. The velocity fields in M-stars strongly depend on log g and Teff. They become stronger with decreasing log g and increasing Teff.