The Structure of the Radio Recombination Line Maser Emission in the
  Envelope of MWC349A
  The Submillimeter Array (SMA) has been used to image the emission from radio
recombination lines of hydrogen at subarcsecond angular resolution from the
young high-mass star MWC349A in the H26$\alpha$, H30$\alpha$, and H31$\alpha$
transitions at 353, 232, and 211 GHz, respectively. Emission was seen over a
range of 80 kms-1 in velocity and 50~mas (corresponding to 60~AU for a distance
of 1200 pc). The emission at each frequency has two distinct components, one
from gas in a nearly edge-on annular disk structure in Keplerian motion, and
another from gas lifted off the disk at distances of up to about 25~AU from the
star. The slopes of the position-velocity (PV) curves for the disk emission
show a monotonic progression of the emission radius with frequency with
relative radii of $0.85\pm0.04$, 1, and $1.02\pm0.01$ for the H26$\alpha$,
H30$\alpha$, and H31$\alpha$ transitions, respectively. This trend is
consistent with theoretical excitation models of maser emission from a region
where the density decreases with radius and the lower transitions are
preferentially excited at higher densities. The mass is difficult to estimate
from the PV diagrams because the wind components dominate the emission at the
disk edges. The mass estimate is constrained to be only in the range of 10--30
solar masses. The distribution of the wind emission among the transitions is
surprisingly different, which reflects its sensitivity to excitation
conditions. The wind probably extracts significant angular momentum from the
system.
