Mid-IR and radio interferometry of S Ori

The dust shell can best be modeled with Al2O3 grains alone using phase-dependent inner boundary radii between 1.8 and 2.4 photospheric radii. The dust shell appears to be more compact with larger optical depth near visual minimum (tau_V_~2.5), and more extended with lower optical depth after visual maximum (tau_V_~1.5). The ratios of the 43.1GHz/42.8GHz SiO maser ring radii to the photospheric radii are 2.2+/-0.3/2.1+/-0.2 (phase 0.44), 2.4+/-0.3/2.3+/-0.4 (0.55), and 2.1+/-0.3/1.9+/-0.2 (1.15).

The maser spots mark the region of the molecular atmospheric layers shortly outward of the steepest decrease of the mid-infrared model intensity profile. Their velocity structure indicates a radial gas expansion. S Ori shows significant phase-dependences of photospheric radii and dust shell parameters.

Al2O3 dust grains and SiO maser spots form at relatively small radii of ~1.8-2.4 photospheric radii. Our results suggest increased mass-loss and dust formation close to the surface near minimum visual phase, when Al2O3 dust grains are co-located with the molecular gas and the SiO maser shells, and a more expanded dust shell after visual maximum. Silicon does not appear to be bound in dust, as our data shows no sign of silicate grains.