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The kinematics of circumstellar disks and disk winds are poorly understood due to the difficulty of producing well resolved observational data. The bright hydrogen recombination-line maser emission originating from the circumstellar disk of MWC 349A offers a unique opportunity to study the disk at milli-arcsecond precision. Using high angular resolution observations of the maser emission from MWC 349A carried out by the SMA, we were able to produce and analyze rotation curves for the H26α, H30α, and H31α transitions. We found that maser features originating from the disk follow Keplerian motion. Furthermore, the H31α masers in the disk appear to form in a narrow annulus at a fixed radius from the star, consistent with previous studies of the H30α and H26α masers. Based on analysis of the rotation curves for the three maser transitions, there appears to be an inverse relationship between the frequency of the maser emission and the radial extent to which the corresponding maser emission spans. This implies that maser transitions for lower quantum numbers occur in the inner and denser regions of the disk than the higher quantum transitions. Using the scaling relationship between quantum number and radius of emission for each maser transition, we derived the density distribution throughout the disk, which follows the relation n e ∼ R −4.9±0.6 . Finally, we found that a stellar mass of M = 10 ± 3 M ? was most consistent with the kinematices of the maser features originating from the Keplerian disk

Using Hydrogen Recombination Masers to Study Disk and Wind Kinematics in MWC 349A