SMA millimeter observations of Hot Molecular Cores
  We present Submillimeter Array observations, in the 1.3 mm continuum and the
CH_3CN(12-11) line of 17 hot molecular cores associated with young high-mass
stars. The angular resolution of the observations ranges from 1".0 to 4".0. The
continuum observations reveal large (>3500 AU) dusty structures with gas masses
from 7 to 375 Msun, that probably surround multiple young stars. The CH_3CN
line emission is detected toward all the molecular cores at least up to the
K=6-component and is mostly associated with the emission peaks of the dusty
objects. We used the multiple K-components of the CH_3CN and both the
rotational diagram method and a simultaneous synthetic LTE model with the
XCLASS program to estimate the temperatures and column densities of the cores.
For all sources, we obtained reasonable fits from XCLASS by using a model that
combines two components: an extended and warm envelope, and a compact hot core
of molecular gas, suggesting internal heating by recently formed massive stars.
The rotational temperatures lie in the range of 40-132 K and 122-485 K for the
extended and compact components, respectively. From the continuum and CH_3CN
results, we infer fractional abundances from 10^{-9} to 10^{-7} toward the
compact inner components, that increase with the rotational temperature. Our
results agree with a chemical scenario in which the CH_3CN molecule is
efficiently formed in the gas phase above 100-300 K, and its abundance
increases with temperature.
