We report Very Long Baseline Array ( VLBA ) observations of 43 GHz v =1 , J =1–0 SiO masers in the circumstellar envelope of the M-type semi-regular variable star VX Sgr at 3 epochs during 1999 April–May .
These high-resolution VLBA images reveal a persistent ringlike distribution of SiO masers with a projected radius of \approx 3 stellar radii .
The typical angular size of 0.5 mas for individual maser feature was estimated from two-point correlation function analysis for maser spots .
We found that the apparent size scale of maser features was distinctly smaller than that observed in the previous observations by comparing their fractions of total power imaged .
This change in the size scale of maser emission may be related to stellar activity that caused a large SiO flare during our observations .
Our observations confirmed the asymmetric distribution of maser emission , but the overall morphology has changed significantly with the majority of masers clustering to the north-east of the star compared to that lying to the south-west direction in 1992 .
By identifying 42 matched maser features appearing in all the three epochs , we determined the contraction of an SiO maser shell toward VX Sgr at a proper motion of -0.507 \pm 0.069 mas yr ^ { -1 } , corresponding to a velocity of about 4 km s ^ { -1 } 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 explained by the gravitational infall of material from the circumstellar dust shell .