Understanding Spatial and Spectral Morphologies of Ultracompact H II
  Regions
  The spatial morphology, spectral characteristics, and time variability of
ultracompact H II regions provide strong constraints on the process of massive
star formation. We have performed simulations of the gravitational collapse of
rotating molecular cloud cores, including treatments of the propagation of
ionizing and non-ionizing radiation. We here present synthetic radio continuum
observations of H II regions from our collapse simulations, to investigate how
well they agree with observation, and what we can learn about how massive star
formation proceeds. We find that intermittent shielding by dense filaments in
the gravitationally unstable accretion flow around the massive star leads to
highly variable H II regions that do not grow monotonically, but rather
flicker, growing and shrinking repeatedly. This behavior appears able to
resolve the well-known lifetime problem. We find that multiple ionizing sources
generally form, resulting in groups of ultracompact H II regions, consistent
with observations. We confirm that our model reproduces the qualitative H II
region morphologies found in surveys, with generally consistent relative
frequencies. We also find that simulated spectral energy distributions (SEDs)
from our model are consistent with the range of observed H II region SEDs,
including both regions showing a normal transition from optically thick to
optically thin emission, and those with intermediate spectral slopes. In our
models, anomalous slopes are solely produced by inhomogeneities in the H II
region, with no contribution from dust emission at millimeter or submillimeter
wavelengths. We conclude that many observed characteristics of ultracompact H
II regions appear consistent with massive star formation in fast,
gravitationally unstable, accretion flows.
