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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-wavelengthtransitions of C3H2 and C2H. In addition, we have traced the distribution ofother molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together withthe reactive ions recently detected, have been considered to determine thephysical conditions and to model the PDR chemistry. We then identified two kindof molecules. The first group, formed by the reactive ions (CO+, HOC+) andsmall hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and ispresumably exposed to a high UV field (hence we called it as "high UV", orHUV). HUV species is expected to dominate for visual absorptions 2 < 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 gasphase models, this is not the case for the studied LUV ones. Although someefficient gas-phase reactions might be lacking, grain chemistry sounds like aprobable mechanism able to explain the observed enhancement of HCO and SiO.Within this scenario, the interaction of UV photons with grains produces animportant effect on the molecular gas chemistry and constitutes the firstevidence of an ionization front created by the UC HII region carving its hostmolecular cloud. The physical conditions and kinematics of the gas layer whichsurrounds the UC HII region were derived from the HUV molecules. Molecularhydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations.Such high densities suggest that the HII region could be pressure-confined bythe surrounding high density molecular gas.Comment: 32 pages, 8 figures. Accepted by Astrophysical Journa

Insights into the Carbon Chemistry of Monoceros R2