On the observability of geometric pulsation of M-type Mira variables

The geometric pulsation of an M-type Mira variable as a function of phase is studied by means of a series of complete self-excited models that follows two successive cycles with time intervals of about 0.1 cycles. Pulsation of the optical-depth diameter in a real-continuum bandpass in the near-infrared is predicted to be a simple sinusoidal-like motion. Pulsation of interferometrically measurable diameters is close to this motion only in the case of very small molecular-band contamination of the continuum in the bandpass of observation. Contamination depends on phase and on cycle and tends to be smallest just before and after maximum visual phase. The observed phase dependence of near-continuum diameters quoted in the literature results from superposition of effects of geometric pulsation and molecular absorption. Measurements of such molecule-affected diameters are a good tool for probing the time-dependent structure of the upper Mira atmosphere where molecules form.