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Mon R2, at a distance of 830 pc, is the only ultracompact HII region (UC HII)where the photon-dominated region (PDR) between the ionized gas and themolecular cloud can be resolved with Herschel. HIFI observations of theabundant compounds 13CO, C18O, o-H2-18O, HCO+, CS, CH, and NH have been used toderive the physical and chemical conditions in the PDR, in particular the waterabundance. The 13CO, C18O, o-H2-18O, HCO+ and CS observations are welldescribed assuming that the emission is coming from a dense (n=5E6 cm-3,N(H2)>1E22 cm-2) layer of molecular gas around the UC HII. Based on ouro-H2-18O observations, we estimate an o-H2O abundance of ~2E-8. This is theaverage ortho-water abundance in the PDR. Additional H2-18O and/or water linesare required to derive the water abundance profile. A lower density envelope(n~1E5 cm-3, N(H2)=2-5E22 cm-2) is responsible for the absorption in the NH1_1-0_2 line. The emission of the CH ground state triplet is coming from bothregions with a complex and self-absorbed profile in the main component. Theradiative transfer modeling shows that the 13CO and HCO+ line profiles areconsistent with an expansion of the molecular gas with a velocity law, v_e =0.5x (r/Rout)^{-1} km/s, although the expansion velocity is poorly constrained bythe observations presented here.

Herschel observations in the ultracompact HII region Mon R2: Water in dense Photon-dominated regions (PDRs)