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Title:
Interferometric Observations of the SiO Masers and Dust Shell of VX Sagittarii
Authors:
Greenhill, L. J.; Colomer, F.; Moran, J. M.; Backer, D. C.; Danchi, W. C.; Bester, M.
Publication:
Astrophysical Journal v.449, p.365 (ApJ Homepage)
Publication Date:
08/1995
Origin:
APJ; KNUDSEN
Astronomy Keywords:
MASERS, STARS: CIRCUMSTELLAR MATTER, STARS: INDIVIDUAL CONSTELLATION NAME: VX SAGITTARII, TECHNIQUES: INTERFEROMETRIC
DOI:
10.1086/176060
Bibliographic Code:
1995ApJ...449..365G

Abstract

We report the results of VLBI observations at 7 mm wavelength of the SiO maser emission (J = 1 → 0, υ = 1) in the circumstellar envelope of the late-type variable star VX Sagittarii. These are the first VLBI observations that have been coordinated with mid-infrared interferometric observations of late-type stars. Synthesis images show that the maser emission at a stellar phase of about 0.4 arises from a ringlike distribution with a radius of about 1.3 R*. The distribution of emission is asymmetric and the strongest emission arises from the redshifted center of activity that lies to the south of the star. This may indicate an asymmetry in the mass-loss or stellar atmosphere. The maser emission within 4 km s-1 of the stellar velocity is distributed around the stellar limb and does not show evidence of systematic velocity gradients greater than a few km s-1. The overlap of maser features is consistent with outflow (or infall) of material away from (or toward) the star of about 10 km s-1 at 1.3 R*, about half the velocity 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 dense velocity coherent structures with characteristic sizes of ˜0.4 AU (0.02 R*) in the extended atmosphere above the photosphere. 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 velocity 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 measured with the UC Berkeley Infrared Spatial Interferometer at about the same stellar minimum. The stellar radius at stellar minimum was measured to be 22±3 AU (13±2 mas). Infrared observations made close 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 masers, and the density at which the maser levels probably thermalize are consistent with models of atmospheric extension due to pulsation-driven shocks.

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