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