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 ^10.2mas at phase 0.95. 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. ; Pub. in The Astrophysical Jnl., v618 p953-961, 10 Jan 2005. Prepared in cooperation with the European Southern Observatory, Munich, Germany. The original document contains color images.