Dynamical models of Mira atmospheres: shocks, limb functions, and Mg II emission

Dynamic atmosphere models of Mira-type stars, prepared using a code developed by G. H. Bowen, were analyzed to determine observational implications of the models and suggest improvements to the code and model parameters. Three specific areas were addressed: Shock morphology, limb functions, and Mg II emission;The long-period, fundamental-mode models used in this study exhibit an unexpected shock morphology. In addition to the main shock, which forms as the radially pulsating surface of the Mira moves outward and is observed to travel out through atmosphere, a preliminary shock structure forms as rebounding layers of the atmosphere fall back onto lower layers. The preliminary shock remains deep in the atmosphere until overrun by the outward-moving main shock. The energy dissipated by the preliminary shock usually exceeds that dissipated by the main shock, and has important effects on the light curve;The dynamic atmosphere models exhibit radial extension of the atmosphere and post-shock emission that alter the limb function (limb darkening/brightening) of the models. The effects on stellar angular diameters measured by lunar occultation technique are calculated. The results show that the usual procedure of fitting occultation observations assuming a uniform brightness disk and then correcting the resulting diameter for limb darkening can give erroneous results. The dynamic effects cause Miras to appear larger and cooler than they actually are;A post-shock relaxation zone code developed by J. N. Pierce was modified and interfaced with the Bowen code to follow the ionization state and cooling radiation emitted by Hydrogen, Helium, and 23 metals in the models. Mg II emission data were used to prepare a light curve that is compared with Mg II light curves observed with the IUE satellite. The relaxation models show that the periodic passage of shocks through the atmosphere results in much lower concentrations of molecular hydrogen and higher ionization fractions for metals than would be predicted by static atmosphere models. The models also indicate that the cooling functions used in the Bowen code should be modified to yield faster high-temperature cooling. The models generated by the Bowen code currently exhibit hot post-shock regions that are far thicker than the relaxation models indicate