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Observations of hypercompact HII (HCHII) regions are not easily explained
by the standard model of HII regions whose dynamics and evolution are
dominated by thermal or turbulent pressure, but these observations are
simply understood within the contexts of more recent models -- hot
molecular cores, quenched HII regions, gravitationally trapped HCHII
regions, photo-evaporating disks, and champagne flows --
that include the gravitational attraction of embedded stars.
These different theoretical models and the different observed
structures of HCHII regions may be unified by a single model in which the
HCHII region develops in an accretion flow that is subject to ionizing
radiation. The differences in the structures of HCHII regions result
from the differences in the structures of the accretion flows that
form with a particular mass, gas density, and angular momentum,
and the flux of ionizing radiation. The HCHII regions and some
ultracompact HII (UCHII) regions are a distinct class of HII
regions whose structure, dynamics, and evolution are dominated not
by pressure but by the gravitational attraction of the embedded
stars and the structures associated with accretion.
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