Context. Monoceros R2, at a distance of 830 pc, is the only ultracompact Hii region (UC H_(II)) 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 (G_0 > 10^5 in units of Habing field, n > 10^6 cm^9−3)).
Aims. Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC H_(II) Mon R2.
Methods. HIFI observations of the abundant compounds ^(13)CO, C^(18)O, o-H_2^(18)O, 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 ^(13)CO, C^(18)O, o-H^(18)_2O, HCO^+ and CS observations are well described assuming that the emission is coming from a dense (n =
5 × 10^6 cm^(−3), N(H_2) > 10^(22) cm^(−2)) layer of molecular gas around the H_(II) region. Based on our o-H^(18)_2O observations, we estimate an o-H_2O
abundance of ≈2 × 10^(−8). This is the average ortho-water abundance in the PDR. Additional H^(18)_2O and/or water lines are required to derive the
water abundance profile. A lower density envelope (n ~ 10^5 cm^(−3), N(H_2) = 2−5 × 10^(22) cm^(−2)) is responsible for the absorption in the NH 1_1 → 0_2
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 ^(13)CO and HCO^+ line profiles are consistent with an expansion of the molecular gas with a velocity
law, v_e = 0.5 × (r/R_(out))^(−1) km s^(−1), although the expansion velocity is poorly constrained by the observations presented here.
Conclusions. We determine an ortho-water abundance of ≈2 × 10^(−8) in Mon R2. Because shocks are unimportant in this region and our estimate is
based on H^(18)_2O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far
