VLBI imaging of the 86 GHz SiO maser emission in the circumstellar envelope of VX Sagittarii

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 similar to 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 similar to 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 similar to 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.