Massive Star Formation of the Sgr A East HII Regions Near the Galactic
  Center
  A group of four compact HII regions associated with the well-known 50 km/s
molecular cloud is the closest site of on-going star formation to the dynamical
center of the Galaxy, at a projected distance of ~6 pc. We present a study of
ionized gas based on the [NeII] (12.8 micron) line, as well as multi-frequency
radio continuum, HST Pa alpha and Spitzer IRAC observations of the most compact
member of the HII group, Sgr A East HII D. The radio continuum image at 6cm
shows that this source breaks up into two equally bright ionized features, D1
and D2. The SED of the D source is consistent with it being due to a 25\pm3
solar mass, star with a luminosity of 8\pm3x10^4 solar luminosity. The inferred
mass, effective temperature of the UV source and the ionization rate are
compatible with a young O9-B0 star. The ionized features D1 and D2 are
considered to be ionized by UV radiation collimated by an accretion disk. We
consider that the central massive star photoevaporates its circumstellar disk
on a timescale of 3x10^4 years giving a mass flux ~3x10^{-5} solar mass per yr
and producing the ionized material in D1 and D2 expanding in an inhomogeneous
medium. The ionized gas kinematics, as traced by the [Ne II] emission, is
difficult to interpret, but it could be explained by the interaction of a
bipolar jet with surrounding gas along with what appears to to be a conical
wall of lower velocity gas. The other HII regions, Sgr A East A-C, have
morphologies and kinematics that more closely resemble cometary flows seen in
other compact HII regions, where gas moves along a paraboloidal surface formed
by the interaction of a stellar wind with a molecular cloud.
