Letter to the Editor

Herschel observations in the ultracompact HII region Mon R2^*,**

Water in dense photon-dominated regions (PDRs)

¹ Observatorio Astronómico Nacional (OAN), Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain e-mail: a.fuente@oan.es
² Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Centro de Astrobiología, CSIC-INTA, 28850 Madrid, Spain
⁴ Instituto de Radio Astronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁵ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31062 Toulouse Cedex 4, France
⁶ CNRS, UMR 5187, 31028 Toulouse, France
⁷ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁸ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁹ LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
¹¹ Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
¹² Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
¹³ Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁴ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
¹⁵ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁶ Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen, 92190 Meudon, France 

¹⁷ IPAC/Caltech, MS 100-22, Pasadena, CA 91125, USA
¹⁸ Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India
¹⁹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
²⁰ Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint-Martin d'Hères, France
²¹ California Institute of Technology, 320-47, Pasadena, CA 91125-4700, USA
²² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²³ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
²⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁵ Institut für 4D-Technologien, FHNW, 5210 Windisch, Switzerland
²⁶ Osservatorio Astrofisico di Arcetri-INAF, Largo E. Fermi 5, 50100 Florence, Italy
²⁷ N. Copernicus Astronomical Center, Rabianska 8, 87-100, Torun, Poland
²⁸ Department of Physics and Astronomy, University of Waterloo, Waterloo, ON N2L 3G1, Canada
²⁹ University of Western Ontario, Dept. of Physics & Astronomy, London, Ontario, N6A 3K7, Canada
³⁰ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands

Received: 31 May 2010
Accepted: 14 June 2010

Abstract

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 (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 H ii Mon R2.

Methods. HIFI observations of the abundant compounds ¹³CO, C¹⁸O, o-H₂¹⁸O, 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-H₂¹⁸O, HCO⁺ and CS observations are well described assuming that the emission is coming from a dense (n = 5 × 10⁶ cm^-3, N(H₂)> 10²² cm^-2) layer of molecular gas around the H ii region. Based on our o-H₂¹⁸O observations, we estimate an o-H₂O abundance of ≈2 × 10^-8. 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 (n ~ 10⁵ cm^-3, N(H₂) = 2–5 × 10²² cm^-2) is responsible for the absorption in the NH 1_{1 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 ¹³CO 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 H₂¹⁸O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far.