SiO masers in Mira variables at a single stellar phase

A hydrodynamic pulsation model for long-period M-type Mira variables is used to estimate the physical conditions of number density, temperature and velocity field in the circumstellar envelope of a model star of 1 M⊙, 332-d period, surface temperature 3000 K and mass-loss rate 1.8 × 10-7 M⊙ yr-1. These conditions have previously been used in a model of SiO masers to establish, for a single stellar phase, the nature and location of the zone in which SiO masers should form. Using Monte Carlo techniques, a synthetic image of 43-GHz emission is generated, showing that masers are predicted to form in an approximate ring 1 to 2 stellar radii from the photosphere, in agreement with the results of recent very long baseline interferometry (VLBI) observations. A new image of SiO masers around TX Cam, obtained using the Very Long Baseline Array (VLBA), is presented, which shows these characteristics. Models show that masers do not form in the region in which the stellar wind is fully developed. Amplification is dominated by tangential paths through the limbs of the circumstellar envelope. Synthetic lineshapes for a variety of maser transitions show structure and linewidths that are typical of observed lineshapes.