Isolated Massive Star Formation in G28.20-0.05
  We report high-resolution 1.3mm continuum and molecular line observations of
the massive protostar G28.20-0.05 with ALMA. The continuum image reveals a
ring-like structure with 2,000 au radius, similar to morphology seen in
archival 1.3cm VLA observations. Based on its spectral index and associated
H30$\alpha$ emission, this structure mainly traces ionized gas. However, there
is evidence for $\sim$30 M$\odot$ of dusty gas near the main mm continuum peak
on one side of the ring, as well as in adjacent regions within 3,000 au. A
virial analysis on scales of $\sim$2,000 au from hot core line emission yields
a dynamical mass of $\sim$80M$\odot$. A strong velocity gradient in the
H30$\alpha$ emission is evidence for a rotating, ionized disk wind, which
drives a larger-scale molecular outflow. An infrared SED analysis indicates a
current protostellar mass of m$_{star}\sim$24 M$\odot$ forming from a core with
initial mass $M_c\sim400\:M_\odot$ in a clump with mass surface density of
$\Sigma_{\rm cl}\sim 3\:{\rm g\:cm}^{-2}$. Thus the SED and other properties of
the system can be understood in the context of core accretion models.
Structure-finding analysis on the larger-scale continuum image indicates
G28.20-0.05 is forming in a relatively isolated environment, with no other
concentrated sources, i.e., protostellar cores, above $\sim$ 1 M$\odot$ found
from $\sim$0.1 to 0.4 pc around the source. This implies that a massive star is
able to form in relative isolation and the dearth of other protostellar
companions within the $\sim$1 pc environs is a strong constraint on massive
star formation theories that predict the presence of a surrounding
protocluster.
