Aiming to learn about the chemistry of the dense PDR around the ultracompact ( UC ) H ii region in ( catalog 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 “ high UV ” , or HUV ) .
HUV species is expected to dominate for visual absorptions 2 < A _ { \mathrm { V } } < 5 mag .
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 ( 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 H ii region carving its host molecular cloud .
The physical conditions and kinematics of the gas layer which surrounds the UC H ii 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 H ii region could be pressure-confined by the surrounding high density molecular gas .