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We used the Submillimeter Array to map the angular distribution of theH30$\alpha$ recombination line (231.9 GHz) in the circumstellar region of thepeculiar star MWC349A. The resolution was $1\farcs2$, but because of highsignal-to-noise ratio we measured the positions of all maser components toaccuracies better than $0\farcs01$, at a velocity resolution of $1 kms$. Thetwo strongest maser components (called high velocity components) at velocitiesnear -14 and $32 kms$ are separated by $0\farcs048 \pm 0\farcs001$ (60 AU)along a position angle of $102 \pm 1\arcdeg$. The distribution of maseremission at velocities between and beyond these two strongest components werealso provided. The continuum emission lies at the center of the maserdistribution to within 10 mas. The masers appear to trace a nearly edge-onrotating disk structure, reminiscent of the water masers in Keplerian rotationin the nuclear accretion disk of the galaxy NGC4258. However, the masercomponents in MWC349A do not follow a simple Keplerian kinematic prescriptionwith $v \sim r^{-1/2}$, but have a larger power law index. We explore thepossibility that the high velocity masers trace spiral density or shock waves.We also emphasize caution in the interpretation of relative centroid maserpositions where the maser is not clearly resolved in position or velocity, andwe present simulations that illustrate the range of applicability of thecentroiding method.

SMA Imaging of the Maser Emission from the H30$\alpha$ Radio Recombination Line in MWC349A