Three-dimensional simulations of near-surface convection in main-sequence stars - I. Overall structure

TL;DR

This study uses 3D radiation hydrodynamics simulations to analyze the structure of near-surface convection in main-sequence stars, revealing systematic trends and a transition in granulation properties across spectral types.

Contribution

It provides a comprehensive parameter study of convection properties in main-sequence stars from F3V to M2V, highlighting differences and transitions in surface convection mechanisms.

Findings

Monotonic trends in granule size, flow velocity, and intensity contrast along the main sequence.

Significant differences in convection between M stars and hotter stars due to energy transport transition.

Identification of a transition from 'naked' to 'hidden' granulation around spectral type K5V.

Abstract

The near-surface layers of cool main-sequence stars are structured by convective flows, which are overshooting into the atmosphere. The flows and the associated spatio-temporal variations of density and temperature affect spectral line profiles and thus have an impact on estimates of stellar properties such as effective temperature, gravitational acceleration, and abundances. We aim at identifying distinctive properties of the thermodynamic structure of the atmospheres of different stars and understand their causes. We ran comprehensive 3D radiation hydrodynamics simulations of the near-surface layers of six simulated stars of spectral type F3V to M2V with the MURaM code. We carry out a systematic parameter study of the mean stratifications, flow structures, and the energy flux in these stars.\par Results: We find monotonic trends along the lower main sequence in granule size, flow velocity, and intensity contrast. The convection in the M-star models differs substantially from that of the hotter stars, mainly owing to the more gradual transition from convective to radiative energy transport. While the basic mechanisms driving surface convection in cool stars are the same, the properties of the convection vary along the main sequence. Apart from monotonic trends in rms velocity, intensity contrast, granule size, etc., there is a transition between "naked" and "hidden" granulation around spectral type K5V caused by the (highly non-linear) temperature dependence of the opacity. These variations have to be taken into account when stellar parameters are derived from spectra.