VLBI Imaging of the 86 GHz SiO Maser Emission in the Circumstellar Envelope of VX Sagittarii

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Abstract
We have observed and imaged the SiO maser source in the late-type star VX Sgr with a single VLBI baseline at 3.5 mm wavelength. Phase-referenced channel maps with a beamwidth of ~0.4 mas show multiple components over a LSR velocity range of -1 to 24 km s-1, and many of the maser features display velocity widths smaller than 1 km s-1. The maser spots in this 86 GHz transition (v = 1, J = 2 → 1) are arranged in an irregular ring-shaped configuration of diameter ~30 mas, echoing the structures found in the J = 1 → 0 transition 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-1 among the strongest complex of masers on the southernmost portion of the ring structure. This velocity gradient can be interpreted as a rigid rotation of the circumstellar envelope with v sin i ~ 13 km s-1. Maser emission is not detected on long (5000 km) baselines, which places a 0.1 mas lower limit on the size of strong maser spots.
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