Modeling the near-UV band of GK stars, Paper II: NLTE models

TL;DR

This paper develops and compares NLTE and LTE atmospheric models for GK stars, showing NLTE models provide more accurate stellar parameters and better spectral fits, especially for giants, improving stellar atmosphere modeling accuracy.

Contribution

First detailed analysis of NLTE effects on the thermal structure and spectral energy distributions of GK stars, enhancing stellar atmosphere models.

Findings

NLTE models yield lower Teff by 30-90 K for giants.

NLTE models improve consistency of log g and Teff estimates.

Blue and red spectral fits are still challenging, especially for giants.

Abstract

We present a grid of atmospheric models and synthetic spectral energy distributions (SEDs) for late-type dwarfs and giants of solar and 1/3 solar metallicity with many opacity sources computed in self-consistent Non-Local Thermodynamic Equilibrium (NLTE), and compare them to the LTE grid of Short & Hauschildt (2010) (Paper I). We describe, for the first time, how the NLTE treatment affects the thermal equilibrium of the atmospheric structure (T(tau) relation) and the SED as a finely sampled function of Teff, log g, and [A/H] among solar metallicity and mildly metal poor red giants. We compare the computed SEDs to the library of observed spectrophotometry described in Paper I across the entire visible band, and in the blue and red regions of the spectrum separately. We find that for the giants of both metallicities, the NLTE models yield best fit Teff values that are ~30 to 90 K lower than those provided by LTE models, while providing greater consistency between \log g values, and, for Arcturus, Teff values, fitted separately to the blue and red spectral regions. There is marginal evidence that NLTE models give more consistent best fit Teff values between the red and blue bands for earlier spectral classes among the solar metallicity GK giants than they do for the later classes, but no model fits the blue band spectrum well for any class. For the two dwarf spectral classes that we are able to study, the effect of NLTE on derived parameters is less significant.