Modeling High-Mass Star Formation and Ultracompact HII Regions
  Massive stars influence the surrounding universe far out of proportion to
their numbers through ionizing radiation, supernova explosions, and heavy
element production. Their formation requires the collapse of massive
interstellar gas clouds with very high accretion rates. We discuss results from
the first three-dimensional simulations of the gravitational collapse of a
massive, rotating molecular cloud core that include heating by both
non-ionizing and ionizing radiation. Local gravitational instabilities in the
accretion flow lead to the build-up of a small cluster of stars. These
lower-mass companions subsequently compete with the high-mass star for the same
common gas reservoir and limit its overall mass growth. This process is called
fragmentation-induced starvation, and explains why massive stars are usually
found as members of high-order stellar systems. These simulations also show
that the HII regions forming around massive stars are initially trapped by the
infalling gas, but soon begin to fluctuate rapidly. Over time, the same
ultracompact HII region can expand anisotropically, contract again, and take on
any of the observed morphological classes. The total lifetime of HII regions is
given by the global accretion timescale, rather than their short internal
sound-crossing time. This solves the so-called lifetime problem of ultracompact
HII region. We conclude that the the most significant differences between the
formation of low-mass and high-mass stars are all explained as the result of
rapid accretion within a dense, gravitationally unstable flow.
