INTERFEROMETRIC OBSERVATIONS OF THE SIO MASERS AND DUST SHELL OF VX-SAGITTARII

We report the results of VLBI observations at 7 mm wavelength of the S iO maser emission (J = 1 --> 0, upsilon = 1) in the circumstellar enve lope of the late-type variable star VX Sagittarii. These are the first VLBI observations that have been coordinated with mid-infrared interf erometric observations of late-type stars. Synthesis images show that the maser emission at a stellar phase of about 0.4 arises from a ringl ike distribution with a radius of about 1.3 R(). The distribution of emission is asymmetric and the strongest emission arises from the reds hifted center of activity that lies to the south of the star. This may indicate an asymmetry in the mass-loss or stellar atmosphere. The mas er emission within 4 km s(-1) of the stellar velocity is distributed a round the stellar limb and does not show evidence of systematic veloci ty gradients greater than a few km s(-1), The overlap of maser feature s is consistent with outflow (or infall) of material away from (or tow ard) the star of about 10 km s(-1) at 1.3 R(), about half the velocit y observed in the OH and H2O maser shells at radii of greater than 30 R(). The arrangement of the maser emission suggests the presence of d ense velocity coherent structures with characteristic sizes of similar to 0.4 AU (0.02 R()) in the extended atmosphere above the photospher e. However, 74% of the maser flux was resolved by these observations, as with earlier, single-baseline VLBI observations of coarser angular resolution. We speculate that the circumstellar shell also has velocit y coherent cells on spatial scales of 30 to perhaps 100 AU, which give rise to SiO maser emission. The compact SiO maser emission lies well within the 4.6 R() inner radius of the circumstellar dust shell, as m easured with the UC Berkeley Infrared Spatial Interferometer at about the same stellar minimum. The stellar radius at stellar minimum was me asured to be 22 +/- 3 AU (13 +/- 2 mas). Infrared observations made cl ose to stellar maximum show an inner radius of 6.0 R(). The estimated density at the inner edge of the dust shell, the location of the mase rs, and the density at which the maser levels probably thermalize are consistent with models of atmospheric extension due to pulsation-drive n shocks.