Inward Motions of the Compact SiO Masers around VX Sagittarii

We report Very Long Baseline Array (VLBA) observations of 43 GHz v=1, J=1-0SiO masers in the circumstellar envelope of the M-type semi-regular variablestar VX Sgr at 3 epochs during 1999 April-May. These high-resolution VLBAimages reveal a persistent ringlike distribution of SiO masers with a projectedradius of ~3 stellar radii. The typical angular size of 0.5 mas for individualmaser feature was estimated from two-point correlation function analysis formaser spots. We found that the apparent size scale of maser features wasdistinctly smaller than that observed in the previous observations by comparingtheir fractions of total power imaged. This change in the size scale of maseremission may be related to stellar activity that caused a large SiO flareduring our observations. Our observations confirmed the asymmetric distributionof maser emission, but the overall morphology has changed significantly withthe majority of masers clustering to the north-east of the star compared tothat lying to the south-west direction in 1992. By identifying 42 matched maserfeatures appearing in all the three epochs, we determined the contraction of anSiO maser shell toward VX Sgr at a proper motion of -0.507 mas/yr,corresponding to a velocity of about 4 km/s at a distance of 1.7 kpc to VX Sgr.Such a velocity is on the order of the sound speed, and can be easily explainedby the gravitational infall of material from the circumstellar dust shell.Comment: 26 pages, 5 figures, 4 tables, accepted for publication in Ap