1996A&A...307..481B

Nonlinear pulsation models designed to represent the prototype Mira variable ο Ceti have been produced. Both fundamental mode and first-overtone models were examined. The fundamental mode models have light and velocity amplitudes similar to those observed and they exhibit bumps on the rising parts of the light curve and aperiodic motion in the upper layers as found observationally in Mira variables. As first found in previous studies, the first-overtone models were not able to produce the observed velocity amplitudes of Mira variables, largely because of the low gravity of the models. This problem suggests that Miras are fundamental mode pulsators. The effect of radiation pressure acting on H₂O molecules in the outer stellar layers was examined as a source of driving mass loss. It was found that the radiation force on H₂O molecules had very little effect on the pulsation and did not lead to any mass loss. A series of photospheric models were made based on the density stratifications of the pulsation models at different phases. Synthetic spectra were computed and these were used to derive monochromatic radii to compare with observations of Mira variables with different periods. The computed radii for τ_λ=1 at different phases in continuum windows was very close to the Rosseland radii. The general appearance of the synthetic spectra and their variation with phase resembled the observations of Mira variables quite well. The behaviour of absorption line profiles from the moving atmospheres was followed through the different phases and cycles of the different model series. A complex behaviour of some lines with phase is predicted. Better agreement between theoretical and observed spectra, colors and radii of Mira variables can be expected to follow from improvements to the treatment of the molecular line opacity.