Axial Symmetry and Rotation in the SiO Maser Shell of IK Tauri

We observed v=1, J=1-0 43-GHz SiO maser emission toward the Mira variable IKTauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resultingfrom these observations show that SiO masers form a highly elliptical ring ofemission approximately 58 x 32 mas with an axial ratio of 1.8:1. The major axisof this elliptical distribution is oriented at position angle of ~59 deg. Theline-of-sight velocity structure of the SiO masers has an apparent axis ofsymmetry consistent with the elongation axis of the maser distribution.Relative to the assumed stellar velocity of 35 km/s, the blue- and red-shiftedmasers were found to lie to the northwest and southeast of this symmetry axisrespectively. This velocity structure suggests a NW-SE rotation of the SiOmaser shell with an equatorial velocity, which we determine to be ~3.6 km/s.Such a NW-SE rotation is in agreement with a circumstellar envelope geometryinvoked to explain previous water and OH maser observations. In this geometry,water and OH masers are preferentially created in a region of enhanced densityalong the NE-SW equator orthogonal to the rotation/polar axis suggested by theSiO maser velocities.Comment: 17 Pages, 4 figures (2 color); accepted for publication in Ap