Joint VLBA/VLTI Observations of the Mira Variable S Orionis

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 = 10.8 ± 0.3 mas was obtained, corresponding to a linear radius of R = 2.3 ± 0.5 AU or R = 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.