Fuente, A. and Lord, S. D. and Phillips, T. G. (2010) Herschel observations in the ultracompact HII region Mon R2: Water in dense photon-dominated regions (PDRs). Astronomy and Astrophysics, 521 . Art. No. L23. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20110111-095309519
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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.
|Additional Information:||© 2010 ESO. Received 31 May 2010, Accepted 14 June 2010, Published online 01 October 2010. 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. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri – INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology – MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University - Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, J.P.L., NHSC. This paper was partially supported by Spanish MICINN under project AYA2009-07304 and within the program CONSOLIDER INGENIO 2010, under grant “Molecular Astrophysics: The Herschel and ALMA Era ASTROMOL” (ref.: CSD2009-00038).|
|Subject Keywords:||ISM: structure – ISM: kinematics and dynamics – ISM: molecules – HII regions – submillimeter: ISM|
|Official Citation:||Herschel observations in the ultracompact HII region Mon R2 - Water in dense photon-dominated regions (PDRs) A. Fuente, O. Berné, J. Cernicharo, J. R. Rizzo, M. González-García, J. R. Goicoechea, P. Pilleri, V. Ossenkopf, M. Gerin, R. Güsten, M. Akyilmaz, A. O. Benz, F. Boulanger, S. Bruderer, C. Dedes, K. France, S. García-Burillo, A. Harris, C. Joblin, T. Klein, C. Kramer, F. Le Petit, S. D. Lord, P. G. Martin, J. Martín-Pintado, B. Mookerjea, D. A. Neufeld, Y. Okada, J. Pety, T. G. Phillips, M. Röllig, R. Simon, J. Stutzki, F. van der Tak, D. Teyssier, A. Usero, H. Yorke, K. Schuster, M. Melchior, A. Lorenzani, R. Szczerba, M. Fich, C. McCoey, J. Pearson and P. Dieleman A&A 521 L23 (2010) DOI: 10.1051/0004-6361/201015093|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Benjamin Perez|
|Deposited On:||11 Jan 2011 19:26|
|Last Modified:||26 Dec 2012 12:50|
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