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 IK Tauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resulting from these observations show that SiO masers form a highly elliptical ring of emission approximately 58 × 32 mas with an axial ratio of 1.8 : 1. The major axis of this elliptical distribution is oriented at a position angle of ~59°. The line-of-sight velocity structure of the SiO masers has an apparent axis of symmetry consistent with the elongation axis of the maser distribution. Relative to the assumed stellar velocity of 35 km s-1, the blue- and redshifted masers were found to lie to the northwest and southeast of this symmetry axis, respectively. This velocity structure suggests a northwest-southeast (NW-SE) rotation of the SiO maser shell with an equatorial velocity, which we determine to be ~3.6 km s-1. Such a NW-SE rotation is in agreement with a circumstellar envelope geometry invoked to explain previous H2O and OH maser observations. In this geometry, H2O and OH masers are preferentially created in a region of enhanced density along the NE-SW equator orthogonal to the rotation/polar axis suggested by the SiO maser velocities.