Resumen:
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 enhanceme...