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 km s ^ { -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 \pm 0.04 , 1 , and 1.02 \pm 0.01 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 .