  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.

Astrophysics