Study of the molecular and ionized gas in a possible precursor of an
  ultra-compact HII region
  We study the molecular and the ionized gas in a possible precursor of an UC
HII region to contribute to the understanding of how high-mass stars build-up
their masses once they have reached the zero-age main secuence. We carried out
molecular observations toward the position of the Red MSX source
G052.9221-00.4892, using ASTE in the 12CO, 13CO, and C18O J=3-2, and HCO+ J=4-3
lines. We also present radio continuum observations at 6 GHz carried out with
the JVLA interferometer. Combining these observations with public infrared data
allowed us to inquire about the nature of the source. The analysis of the
molecular observations reveals the presence of a kinetic temperature and H2
column density gradients across the molecular clump in which the source is
embedded, with the hotter and less dense gas in the inner region. The 12CO
J=3-2 emission shows evidence of misaligned massive molecular outflows, with
the blue lobe in positional coincidence with a jet-like feature seen at 8 um.
The radio continuum emission shows a slightly elongated compact radio source in
positional coincidence with the Red MSX source. The polar-like morphology of
this compact radio source perfectly matches the hourglass-like morphology
exhibited by the source in the Ks-band. The axes of symmetry of the radio
source and the near-IR nebula are perfectly aligned. Based on the
multiwavelength analysis, we suggest that the analyzed source could be
transiting a HC HII region phase, in which the young central star emits winds
and ionizing radiation through the poles. By the other hand, according to a
comparison between the Br-gamma and the radio flux density, the source would be
in a more evolved evolutionary stage of an optically thin UC HII region in
photoionization equilibrium. If this is the case, from the radio continuum
emission, we can conjecture upon the spectral type of its exciting star which
would be a B0.5V.
