Insights into the Carbon chemistry of Mon R2

J. R. Rizzo$^1$, A. Fuente$^2$, and S. Garcia-Burillo$^2$

1 - Universidad Europea de Madrid, Spain
2 - Observatorio Astron\'omico Nacional, Spain

Aiming to learn about the chemistry of the dense PDR around the ultracompact (UC)
HII region in Mon R2, we have observed a series of mm-wavelength
transitions of C$_3$H$_2$ and C$_2$H. In addition, we have traced the distribution of
other molecules, such as H$^{13}$CO$^{+}$, SiO, HCO, and HC$_3$N. These data,
together with the reactive ions recently detected, have been considered to
determine the physical conditions and to model the PDR chemistry. We then
identified two kind of molecules. The first group, formed by the reactive ions
(CO$^+$, HOC$^+$) and small hydrocarbons (C$_2$H, C$_3$H$_2$), traces the surface
layers of the PDR and is presumably exposed to a high UV field (hence we called it
as ``{\it high UV}'', or HUV). HUV species is expected to dominate for visual
absorptions $2field, and hence called ``{\it low UV}'', or LUV) includes HCO and SiO, and is
mainly present at the edges of the PDR ($A_{\mathrm V}>5$\,mag). While the
abundances of the HUV molecules can be explained by gas phase models, this is not
the case for the studied LUV ones. Although some efficient gas-phase reactions might
be lacking, grain chemistry sounds like a probable mechanism able to explain the
observed enhancement of HCO and SiO. Within this scenario, the interaction of
UV photons with grains produces an important effect on the molecular gas
chemistry and constitutes the first evidence of an ionization front created by the
UC HII region carving its host molecular cloud. The physical conditions and
kinematics of the gas layer which surrounds the UC HII region were derived
from the HUV molecules. Molecular hydrogen densities $>4\,10^6$ cm$^{-3}$ are
required to reproduce the observations. Such high densities suggest that the HII
region could be pressure-confined by the surrounding high density molecular gas.

Reference: Astrophysical Journal, in press
Status: Manuscript has been accepted