2012A&A...544A.110P

Mon R2, at a distance of 830pc, is the only ultracompact HII region (UCHII) where the associated photon-dominated region (PDR) can be resolved with Herschel. Owing to its brightness and proximity, it is one of the best-suited sources for investigating the chemistry and physics of highly UV-irradiated PDRs. Our goal is to estimate the abundance of H₂O and NH₃ in this region and investigate their origin. We present new observations ([CII], ¹²CO, ¹³CO, C¹⁸O, o-H₂O, p-H₂O, o-H₂¹⁸O and o-NH₃) obtained with the HIFI instrument onboard Herschel and the IRAM-30m telescope. We investigated the physical conditions in which these lines arise by analyzing their velocity structure and spatial variations. Using a large velocity gradient approach, we modeled the line intensities and derived an average abundance of H₂O and NH₃ across the region. Finally, we modeled the line profiles with a non-local radiative transfer model and compared these results with the abundance predicted by the Meudon PDR code. The variations of the line profiles and intensities indicate complex geometrical and kinematical patterns. In several tracers ([CII], CO 9->8 and H₂O) the line profiles vary significantly with position and have broader line widths toward the HII region. The H₂O lines present strong self-absorption at the ambient velocity and emission in high-velocity wings toward the HII region. The emission in the o-H₂¹⁸O ground state line reaches its maximum value around the HII region, has smaller linewidths and peaks at the velocity of the ambient cloud. Its spatial distribution shows that the o-H₂¹⁸O emission arises in the PDR surrounding the HII region. By modeling the o-H₂¹⁸O emission and assuming the standard [¹⁶O]/[¹⁸O]=500, we derive a mean abundance of o-H₂O of ∼10^–8 relative to H₂. The ortho-H₂O abundance, however, is larger (∼1x10^–7) in the high-velocity wings detected toward the HII region. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope of the core with an average abundance of ∼2x10^–9 relative to H₂. The Meudon PDR code, which includes only gas-phase chemical networks, can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a ≲1mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the water and ammonia abundances in the rest of the cloud, the molecular freeze out and grain surface chemistry would need to be included.