The Mira star S Ori: SiO maser shells related to the stellar photosphere, the molecular layers, and the dust shell at three epochs

Abstract

We present recent results from our project of concurrent radio and infrared interferometry of oxygen-rich Mira stars. Our pilot study on the Mira variable S Ori included coordinated VLBA mapping of the v=1, J=1-0 43.1 GHz and v=2, J=1-0 42.8 GHz SiO maser radiation at one epoch and concurrent near-infrared K-band interferometry to constrain the stellar photospheric diameter. We recently obtained new maps of these SiO maser transitions toward S Ori at three additional epochs with the VLBA and concurrent mid-infrared interferometric data with VLTI/MIDI. The MIDI data are analyzed using recent self-excited dynamic model atmospheres including molecular shells close to continuum-forming layers, which we complement by a radiative transfer model of the dust shell. The modelling of our MIDI data results in phase-dependent continuum photospheric angular diameters. The dust shell can best be modelled with Al₂O₃ grains alone. We find that the mean SiO maser ring radii lie between about 1.9 and 2.4 stellar continuum radii. The maser spots mark the region of the molecular atmospheric layers shortly outward of the steepest decrease of the mid-infrared model intensity profile. Our results suggest that the SiO maser shells are co-located with the Al₂O₃ dust shell near minimum visual phase. Their velocity structure indicates a radial gas expansion. Preliminary results from a study of the Mira star GX Mon suggest a similar picture.