Abstract:
We have observed and imaged the SiO maser source in the late-type starVX Sgr with a single VLBI baseline at 3.5 mm wavelength.Phase-referenced channel maps with a beamwidth of ~0.4 mas show multiplecomponents over a LSR velocity range of -1 to 24 km s-1, andmany of the maser features display velocity widths smaller than 1 kms-1. The maser spots in this 86 GHz transition (v = 1, J = 2--> 1) are arranged in an irregular ring-shaped configuration ofdiameter ~30 mas, echoing the structures found in the J = 1 --> 0transition at 43 GHz around several Mira variables and VX Sgr itself.Unlike previous observations of SiO masers, in VX Sgr we detect a clear,systematic velocity gradient of ~0.6 km s-1 AU-1among the strongest complex of masers on the southernmost portion of thering structure. This velocity gradient can be interpreted as a rigidrotation of the circumstellar envelope with v sin i ~ 13 kms-1. Maser emission is not detected on long (5000 km)baselines, which places a 0.1 mas lower limit on the size of strongmaser spots.