<3D> NLTE line formation in the atmospheres of red supergiants

TL;DR

This paper investigates NLTE effects on spectral line formation in red supergiants' atmospheres, improving the accuracy of chemical abundance measurements crucial for studying distant galaxies.

Contribution

It introduces NLTE radiative transfer models for red supergiants, enhancing the precision of metallicity and temperature determinations from J-band spectra.

Findings

NLTE models enable accurate metallicity and temperature derivation.

First NLTE spectra calculations with a 3D model of Betelgeuse.

Discussion on integrating NLTE with 3D hydrodynamical models.

Abstract

Red supergiants with their enormous brightness at J-band are ideal probes of cosmic chemical composition. It is therefore crucial to have realistic models of radiative transfer in their atmospheres, which will permit determination of abundances accurate to 0.15 dex, the precision attainable with future telescope facilities in galaxies as distant as tens of Mpc. Here, we study the effects of non-local thermodynamic equilibrium (NLTE) on the formation of iron, titanium, and silicon lines, which dominate J-band spectra of red supergiants. It is shown that the NLTE radiative transfer models enable accurate derivation of metallicity and effective temperature in the J-band. We also discuss consequences for RSG spectrum synthesis in different spectral windows, including the heavily TiO-blanketed optical region, and atmospheric structure. We then touch upon challenges of NLTE integration with new generation of 3D hydrodynamical RSG models and present the first calculations of NLTE spectra with the mean 3D model of Betelgeuse.