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

Context. Monoceros 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 the molecular cloud can be resolved with Herschel. Therefore, it is an excellent laboratory to study the chemistry in extreme PDRs (G0 > 105 in units of Habing field, n > 106 cm-3). Aims. Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC Hii Mon R2. Methods. HIFI observations of the abundant compounds 13CO, C18O, o-H2 18O, HCO+, CS, CH, and NH have been used to derive the physical and chemical conditions in the PDR, in particular the water abundance. The modeling of the lines has been done with the Meudon PDR code and the non-local radiative transfer model described by Cernicharo et al. Results. The 13CO, C18O, o-H18 2 O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (n = 5 x 106 cm-3, N(H2) > 1022 cm-2) layer of molecular gas around the Hii region. Based on our o-H18 2 O observations, we estimate an o-H2O abundance of similar to 2 x 10-8. This is the average ortho-water abundance in the PDR. Additional H18 2 O and/ or water lines are required to derive the water abundance profile. A lower density envelope (n similar to 105 cm-3, N(H2) = 2-5 x 1022 cm-2) is responsible for the absorption in the NH 11. 02 line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component. The radiative transfer modeling shows that the 13CO and HCO+ line profiles are consistent with an expansion of the molecular gas with a velocity law, ve = 0.5 x (r/ Rout)-1 km s-1, although the expansion velocity is poorly constrained by the observations presented here. Conclusions. We determine an ortho-water abundance of similar to 2 x 10-8 in Mon R2. Because shocks are unimportant in this region and our estimate is based on H18 2 O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far.