Photospheres of Mira Variables

Extract

Mira variables form an important subgroup of the red giant stars which are typical representatives of stars showing burnt material at their surfaces. Since the photospheres of Miras are not in hydrostatic equilibrium but are characterized by spherically very extended density stratifications, their properties and emitted spectra differ substantially from those of non-Miras, and any attempts to analyse Mira spectra by means of conventional techniques must fail. Non-hydrostatic models are needed for analysis work.

We computed a small set of exploratory M type (solar abundances) Mira model photospheres whose density distributions were taken from an improved modification of a pulsation model proposed by wood (1979). Besides the fundamental model parameters, i.e. mass, luminosity and radius (or effective temperature), the parameters of the density stratification (e.g. the heights of the density discontinuities at the shock front positions, or the effective gravity acting between two successive shock fronts) must be treated as free parameters within certain limits, owing to insufficient knowledge of the velocity stratification entering the pulsation model. Typical density and temperature stratifications are shown in Fig. 1 of Scholz (1987). Both CO lines, measuring the outflow and infall velocities of matter between the shock fronts, and selected moleculer band features (colors) were found to react sensitively to adjustments of the model parameters. As an example, Fig.1 shows the differences between CO line profiles computed from the near-maximum model of Fig.1 of scholz (1987) (Fig.1a) and those computed from a model in which the effective gravity between the shock fronts was increased by a factor of 1.5 (Fig. 1b). The same velocity stratification was adopted in of monochromatic radii proves to be a powerful tool of diagnostics of Mira photospheres (Scholz and Takeda 1987; Bessell et al. 1987).