Dynamical Opacity-Sampling Models of Mira Variables. I: Modelling Description and Analysis of Approximations

TL;DR

This paper presents the CODEX models for Mira variable star atmospheres, analyzing the physical and numerical approximations involved, and discusses the robustness of key observable predictions despite these approximations.

Contribution

Introduction of the detailed CODEX dynamical atmosphere models for Mira variables, with analysis of the impact of LTE and grey approximations on observable predictions.

Findings

Infrared diameters and spectra are robust despite approximations.

Visible light is dominated by fluorescence scattering processes.

LTE approximation under-predicts visible fluxes by a factor of two.

Abstract

We describe the Cool Opacity-sampling Dynamic EXtended (CODEX) atmosphere models of Mira variable stars, and examine in detail the physical and numerical approximations that go in to the model creation. The CODEX atmospheric models are obtained by computing the temperature and the chemical and radiative states of the atmospheric layers, assuming gas pressure and velocity profiles from Mira pulsation models, which extend from near the H-burning shell to the outer layers of the atmosphere. Although the code uses the approximation of Local Thermodynamic Equilibrium (LTE) and a grey approximation in the dynamical atmosphere code, many key observable quantities, such as infrared diameters and low-resolution spectra, are predicted robustly in spite of these approximations. We show that in visible light, radiation from Mira variables is dominated by fluorescence scattering processes, and that the LTE approximation likely under-predicts visible-band fluxes by a factor of two.