Non-local thermodynamic equilibrium 1.5D modeling of red giant stars

Abstract

Spectra for two-dimensional (2D) stars in the 1.5D approximation are created from synthetic spectra of one-dimensional (1D) non-local thermodynamic equilibrium (NLTE) spherical model atmospheres produced by the PHOENIX code. The 1.5D stars have the spatially averaged Rayleigh–Jeans flux of a K3–4 III star while varying the temperature difference between the two 1D component models (ΔT_1.5D) and the relative surface area covered. Synthetic observable quantities from the 1.5D stars are fitted with quantities from NLTE and local thermodynamic equilibrium (LTE) 1D models to assess the errors in inferred T_eff values from assuming horizontal homogeneity and LTE. Five different quantities are fit to determine the T_eff of the 1.5D stars: UBVRI photometric colors, absolute surface flux spectral energy distributions (SEDs), relative SEDs, continuum normalized spectra, and TiO band profiles. In all cases except the TiO band profiles, the inferred T_eff value increases with increasing ΔT_1.5D. In all cases, the inferred T_eff value from fitting 1D LTE quantities is higher than from fitting 1D NLTE quantities and is approximately constant as a function of ΔT_1.5D within each case. The difference between LTE and NLTE for the TiO bands is caused indirectly by the NLTE temperature structure of the upper atmosphere, as the bands are computed in LTE. We conclude that the difference between T_eff values derived from NLTE and LTE modeling is relatively insensitive to the degree of the horizontal inhomogeneity of the star being modeled and largely depends on the observable quantity being fit.