Radiative hydrodynamics simulations of red supergiant stars. IV gray versus non-gray opacities

TL;DR

This paper presents advanced 3D radiative-hydrodynamics simulations of red supergiant stars, comparing gray and non-gray opacities, revealing impacts on thermal gradients, spectral features, and the calibration of turbulence parameters in stellar models.

Contribution

It introduces a new generation of red supergiant simulations with non-gray opacities, providing insights into their atmospheric dynamics and improving 1D model calibration using 3D results.

Findings

Non-gray opacities lead to steeper thermal gradients in the optical thin region.

Temperature fluctuations are weaker with non-gray opacities, reducing radius determination uncertainties.

Empirical calibration of turbulence parameters for 1D models based on 3D simulations.

Abstract

Red supergiants are massive evolved stars that contribute extensively to the chemical enrichment of our Galaxy. It has been shown that convection in those stars gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The understanding of their dynamics is crucial to unveil the unknown mass-loss mechanism, their chemical composition and stellar parameters. We present a new generation of red supergiants simulations with a more sophisticated opacity treatment done with 3D radiative- hydrodynamics CO5BOLD. In the code, the coupled equations of compressible hydrodynamics and non-local radiation transport are solved in the presence of a spherical potential. The stellar core is replaced by a special spherical inner boundary condition, where the gravitational potential is smoothed and the energy production by fusion is mimicked by a simply producing heat corresponding to the stellar luminosity. The post-processing radiative transfer code OPTIM3D is used to extract spectroscopic and interferometric observables. We show that the relaxation of the assumption of frequency-independent opacities shows a steeper mean thermal gradient in the optical thin region that affect strongly the atomic strengths and the spectral energy distribution. Moreover, the weaker temperature fluctuations reduce the incertitude on the radius determination with interferometry. We show that 1D models of red supergiants must include a turbulent velocity calibrated on 3D simulations to obtain the effective surface gravity that mimic the effect of turbulent pressure on the stellar atmosphere. We provide an empirical calibration of the ad-hoc micro- and macroturbulence parameters for 1D models using the 3D simulations: we find that there is not a clear distinction between the different macroturbulent profiles needed in 1D models to fit 3D synthetic lines.