2005ApJ...618..953B

We present the first coordinated Very Long Baseline Array (VLBA)/ Very Large Telescope Interferometer (VLTI) measurements of the stellar diameter and circumstellar atmosphere of a Mira variable star. Observations of the v=1, J=1-0 (43.1 GHz) and v=2, J=1-0 (42.8 GHz) SiO maser emission toward the Mira variable S Ori were conducted using the VLBA. Coordinated near-infrared K-band measurements of the stellar diameter were performed using VLTI-VINCI closely spaced in time to the VLBA observations. Analysis of the SiO maser data recorded at a visual variability phase 0.73 shows the average distance of the masers from the center of the distribution to be 9.4 mas for the v=1 masers and 8.8 mas for the v=2 masers. The velocity structure of the SiO masers appears to be random, with no significant indication of global expansion/infall or rotation. The determined near-infrared, K-band, uniform disk (UD) diameters decreased from ∼10.5 mas at phase 0.80 to ∼10.2 mas at phase 0.95. For the epoch of our VLBA measurements, an extrapolated UD diameter of Θ^K_UD=10.8±0.3 mas was obtained, corresponding to a linear radius of R^K_UD=2.3±0.5 AU or R^K_UD=490±115 R_☉. Our coordinated VLBA/VLTI measurements show that the masers lie relatively close to the stellar photosphere at a distance of ∼2 photospheric radii, consistent with model estimates. This result is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase.