Letter to the Editor
Water in dense photon-dominated regions (PDRs)
Observatorio Astronómico Nacional (OAN), Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain e-mail: firstname.lastname@example.org
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
Accepted: 14 June 2010
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 11 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, ve = 0.5 × (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 ≈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.
Key words: ISM: structure / ISM: kinematics and dynamics / ISM: molecules / HII regions / submillimeter: ISM
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Figures 1 and 4 (page 5) are only available in electronic form at http://www.aanda.org
© ESO, 2010