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Circumstellar envelope of VX Sgr resolved with long-baseline interferometry at millimeter and midinfrared wavelengths

We report VLBI observations at 7-mm wavelengths of the SiO maser emission in the circumstellar envelope of the late-type, supergiant variable star VX Sagitarius. Synthesis images, the first ever made of the SiO emission from VX Sgr, show that the maser emission at a stellar phase of about 0.4, arises in a ringlike distribution with a radius of about 1.3 R*. The distribution of emission around the limb is asymmetric, as is the spectrum of emission with respect to the stellar systemic velocity. The strongest emission arises from a redshifted center of activity that lies to the south of the star and may indicate an asymmetry in the stellar atmosphere or in the mass loss. The maser emission within 4 km s-1 of the stellar velocity is distributed along the limb and does not show evidence of systematic velocity gradients greater than a few km s-1. However, there may be evidence of acceleration of material away from the star, where outflow velocities increase from about 10 km s-1 at 1.3 R* to at least 20 km s-1 in the OH and H2O maser shells at radii of about 30 R*. The arrangement of the maser emission supports a model having dense velocity coherent structures with characteristic sizes of approximately 0.5 AU in the extended stellar atmosphere. However, a substantial fraction of the maser flux density has been resolved by these observations, as with earlier VLBI observations of at least three times more coarse angular resolution. We speculate that the circumstellar shell has velocity coherent cells on spatial scales of approximately 0.5 to perhaps 100 AU, which give rise to SiO maser emission. The SiO maser emission lies well within the 4.6 R* inner radius of the surrounding dust shell, as measured with the U. C. Berkeley infrared interferometer. The high angular resolution infrared and millimeter observation were made at similar phases during the same cycle, and thus provide strong evidence that the SiO maser emission arises well away from the dust-rich region of the circumstellar envelope.