SiO masers in TX Cam - Simultaneous VLBA observations of two 43 GHz masers at four epochs

We present the results of simultaneous high resolution observations of v = 1 and v = 2, J = 1- 0 SiO masers toward TX Cam at four epochs covering a stellar cycle. We used a new observing technique to determine the relative positions of the two maser maps. Near maser maximum ( Epochs III and IV), the individual components of both masers are distributed in ring-like structures but the ring is severely disrupted near stellar maser minimum ( Epochs I and II). In Epochs III and IV there is a large overlap between the radii at which the two maser transitions occur. However in both epochs the average radius of the v = 2 maser ring is smaller than for the v = 1 maser ring, the difference being larger for Epoch IV. The observed relative ring radii in the two transitions, and the trends on the ring thickness, are close to those predicted by the model of Humphreys et al. ( 2002, A& A, 386, 256). In many individual features there is an almost exact overlap in space and velocity of emission from the two transitions, arguing against pure radiative pumping. At both Epochs III and IV in many spectral features only 50% of the flux density is recovered in our images, implying significant smooth maser structure. For both transitions we find that red- and blue-shifted masers occur in all parts of the rings, with relatively few masers at the systemic velocity. Thus there is no evidence for rotation, although the blue-shifted masers are somewhat more prominent to the west. At all four epochs red- shifted components are generally brighter than blue-shifted ones. Blue-shifted masers become very weak at some stellar phases but never completely disappear. At Epochs III and IV, we see many filamentary or spoke-like features in both v = 1 and v = 2 masers, especially in the red- shifted gas. These spokes show systematic velocity gradients consistent with a decelerating outward flow with increasing radius. We outline a possible model to explain why, given the presence of these spokes, there is a deficit of maser features at the systemic velocity. The breaking of spherical symmetry by spoke-like features may explain the high-velocity wings seen in SiO maser single dish spectra.