Angular Momentum in Disk Wind Revealed in the Young Star MWC349A
  Disk winds are thought to play a critical role in star birth. As winds
extract excess angular momentum from accretion disks, matter in the disk can be
transported inward to the star to fuel mass growth. However, the observational
evidence of wind carrying angular momentum has been very limited. We present
Submillimeter Array (SMA) observations of the young star MWC349A in the
H26$\alpha$ and H30$\alpha$ recombination lines. The high signal-to-noise
ratios made possible by the maser emission process allow us to constrain the
relative astrometry of the maser spots to a milli-arcsecond precision. Previous
observations of the H30$\alpha$ line with the SMA and the Plateau de Bure
interferometer (PdBI) showed that masers are distributed in the disk and wind.
Our new high resolution observations of the H26$\alpha$ line reveal differences
in spatial distribution from that of the H30$\alpha$ line. H26$\alpha$ line
masers in the disk are excited in a thin annulus with a radius of about 25 AU,
while the H30$\alpha$ line masers are formed in a slightly larger annulus with
a radius of 30 AU. This is consistent with expectations for maser excitation in
the presence of an electron density variation of approximately $R^{-4}$. In
addition, the H30$\alpha$ and H26$\alpha$ line masers arise from different
parts in the wind. This difference is also expected from the maser theory. The
wind component of both masers exhibits line-of-sight velocities that closely
follow a Keplerian law. This result provides strong evidence that the disk wind
extracts significant angular momentum and thereby facilitating mass accretion
in the young star.
