The Circumstellar Envelope of VX Sgr
Session 22 -- Late-Type and Variable Stars
Oral presentation, Monday, 2:30-4:00, Dwinelle 145 Room

[22.03] The Circumstellar Envelope of VX Sgr

L. J. Greenhill, F. Colomer, J. M. Moran (CfA), D. C. Backer, M. A. Bester, W. C. Danchi, and C. H. Townes (U. C. Berkeley)

We have made interferometric observations at millimeter and mid-infrared wavelengths of the late-type supergiant variable star VX Sgr. With the VLBA, we obtained synthesis images of the 7 mm SiO maser emission associated with the circumstellar envelope. Using the U.C. Berkeley Infrared Spatial Interferometer (ISI), we obtained visibility information for the photospheric emission and the circumstellar dust emission at 11.15 $\mu$m, with fringe spacings of 0.6 to 0.2 arcseconds. The ISI is a two element interferometer that employs heterodyne detection of radiation in the 9 to 12 $\mu$m window. The observations were made near minimum luminosity and represent the first time such integrated observations of a circumstellar envelope have been made.

The synthesis imaging shows that the SiO maser emission arises from a region projected along the limb of the star, which is about 2.6 stellar radii across. The distribution of emission around the limb is asymmetric, as is the spectrum of emission with respect to the stellar velocity. The strongest emission arises from a redshifted center of acivity that lies to the south of the star and may indicate an asymmetry in the stellar atmosphere or in the mass-loss from VX Sgr. Apart from the red shifted center of activity, the maser emission is evenly distributed along the limb and does not show evidence of systematic velocity gradients such as would be evident in the presence of rotation.

The structure of the 11 $\mu$m dust emission from the circumstellar shell has been resolved using the ISI. The effective radius at which dust condenses is about 4.6 stellar radii. These observations have provided a direct confirmation of the long-held expectation that SiO masers lie away from the bulk of the circumstellar dust in these sources. By fitting the observed 11 $\mu$m visibility curve to those derived from radiative transfer models of the dust shell, and matching the total modelled flux density to observation, we have estimated the radius of the photosphere, which was used to interpret the SiO maser images.