TY - JOUR 
A1 - Rizzo Caminos, José Ricardo
AU - Fuente, A.
AU - García-Burillo, S.
T1 - Insights into the carbon chemistry of monoceros R2
Y1 - 2005
SN - 0004637X

UR - http://hdl.handle.net/11268/5541
AB - Aiming to learn about the chemistry of the dense PDR around the ultracompact (UC) H ii region in Mon R2,
we have observed a series of millimeter-wavelength transitions of C3H2 and C2H. In addition, we have traced the
distribution of other molecules, such as H13CO+, SiO, HCO, andHC3N. 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 kinds of molecules. The first group, formed by the reactive ions (CO+ and HOC+) and small
hydrocarbons (C2H and C3H2), traces the surface layers of the PDR and is presumably exposed to a high UV field
(hence we call it highUVor HUV).HUVspecies are expected to dominate for visual absorptions 2 mag< AV <5mag.
A second group ( less exposed to the UV field, and hence called low UV, or LUV) includes HCO and SiO and is
mainly present at the edges of the PDR (AV > 5 mag). While the abundances of the HUV molecules can be explained
by gas-phase models, this is not the case for the studied LUVones. 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 ofUV 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 H ii region carving its host
molecular cloud. The physical conditions and kinematics of the gas layer that surrounds the UC H ii region were
derived from the HUV molecules. Molecular hydrogen densities >4 ; 106 cm 3 are required to reproduce the observations.
Such high densities suggest that the H ii region could be pressure-confined by the surrounding high-density
molecular gas.
KW - Astrofísica
KW - Astronomía
KW - Astrofísica
KW - Astronomía
LA - eng
ER -