Forming an O Star via Disk Accretion?
  We present a study of outflow, infall, and rotation in a ~10^5 Lsun (solar
luminosity) star-forming region, IRAS 18360-0537, with Submillimeter Array
(SMA) and IRAM 30m observations. The 1.3 mm continuum map shows a 0.5 pc dust
ridge, of which the central compact part has a mass of ~80 Msun (solar mass)
and harbors two condensations, MM1 and MM2. The CO (2--1) and SiO (5--4) maps
reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC)
with a gas temperature of 320+/-50 K and a mass of ~13 Msun. The outflow has a
gas mass of 54 Msun and a dynamical timescale of 8,000 yr. The kinematics of
the HMC is probed by high-excitation CH3OH and CH3CN lines, which are detected
at sub-arcsecond resolution and unveil a velocity gradient perpendicular to the
outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope
around the HMC is evidenced by CN lines exhibiting a profound inverse P-Cygni
profile, and the estimated mass infall rate, 1.5x10^{-3} Msun/yr, is well
comparable to that inferred from the mass outflow rate. A more detailed
investigation of the kinematics of the dense gas around the HMC is obtained
from the 13CO and C18O (2--1) lines; the position-velocity diagrams of the two
lines are consistent with the model of a free-falling and Keplerian-like
rotating envelope. The observations suggest that the protostar of a current
mass ~10 Msun embedded within MM1 will develop into an O star via disk
accretion and envelope infall.
