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

摘要

Context. Monoceros R2, at a distance of 830 pc, is the only ultracompact H II 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 ( 10^5$ -->G0 > 105 in units of Habing field, n > 106cm-3). Aims. Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC H II MonR2. Methods. HIFI observations of the abundant compounds 13CO, C18O, o-H218O, HCO+,CS, CH, andNH have been used to derive the physical and chemicalconditions in the PDR, in particular the water abundance. The modelingof the lines has been done with the Meudon PDR code and the non-localradiative transfer model described by Cernicharo etal. Results. The 13CO, C18O, o-H218O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (cm-3, N(H 10^{22}$ -->2 )> 1022cm-2) layer of molecular gas around the H II region. Based on our o-H218O observations, we estimate an o-H2O abundance of .This is the average ortho-water abundance in the PDR. Additional H218O and/or water lines are required to derive the water abundance profile. A lower density envelope (cm-3, N(Hcm-2) is responsible for the absorption in the NH line. The emission of the CH ground state triplet is coming from bothregions 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, )-1kms-1, although the expansion velocity is poorly constrained by the observations presented here. Conclusions. We determine an ortho-water abundance of in MonR2. Because shocks are unimportant in this region and our estimate is based on H218Oobservations that avoids opacity problems, this is probably the mostaccurate estimate of the water abundance in PDRs thus far. Key words: ISM: structure - ISM: kinematics and dynamics - ISM: molecules - HII regions - submillimeter: ISM