NLTE 1.5D modelling of red giant stars

Abstract

Spectra for 2D stars in the 1.5D approximation are created from synthetic spectra of 1D non-local thermodynamic equilibrium (NLTE) model atmospheres produced by the PHOENIX code. The 1.5D stars assume the spatially averaged modelling parameters of a standard K3-4 III star, while varying the temperature difference between the two components ( T1:5D). Synthetic observable quantities from the 1.5D stars are fitted with quantities from 1D models to assess the errors in inferred Te values from assuming horizontal homogeneity and local thermodynamic equilibrium (LTE). Five different quantities are fit to determine the Te of the 1.5D stars. In all cases except the TiO bands, the Te value increases with increasing T1:5D, up to 500 K greater than the average of the 1.5D star. In all cases, the inferred Te value from fitting 1D LTE quantities is between 25 K and 150 K higher than from fitting 1D NLTE quantities. This differential modelling study should permit constraints on the extent of Te inhomogeneities in red giants by comparing 1.5D models with observations.