Submillimeter Array Imaging of the Maser Emission from the H30α Radio Recombination Line in MWC 349A
We used the Submillimeter Array to map the angular distribution of the H30α recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC349A. The resolution was 12, but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than 001, at a velocity resolution of 1 kms. The two strongest maser components (called high velocity components) at velocities near -14 and 32 kms are separated by 0048 ± 0001 (60 AU) along a
... on angle of 102 ± 1. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC4258. However, the maser components in MWC349A do not follow a simple Keplerian kinematic prescription with v ∼ r^-1/2, but have a larger power law index. We explore the possibility that the high velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.