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Water Absorption in Galactic Translucent Clouds: Conditions and History of the Gas Derived from Herschel/HIFI PRISMAS Observations

Flagey, N. and Goldsmith, P. F. and Lis, D. C. and Gerin, M. and Neufeld, D. and Sonnentrucker, P. and De Luca, M. and Godard, B. and Goicoechea, J. R. and Monje, R. and Phillips, T. G. (2013) Water Absorption in Galactic Translucent Clouds: Conditions and History of the Gas Derived from Herschel/HIFI PRISMAS Observations. Astrophysical Journal, 762 (1). Art. No. 11. ISSN 0004-637X. doi:10.1088/0004-637X/762/1/11.

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We present Herschel/HIFI observations of the three ground state transitions of H_2O (556, 1669, and 1113 GHz) and H_2^(18)O (547, 1655, and 1101 GHz)—as well as the first few excited transitions of H_2O (987, 752, and 1661 GHz)—toward six high-mass star-forming regions, obtained as part of the PRISMAS (PRobing InterStellar Molecules with Absorption line Studies) Guaranteed Time Key Program. Water vapor associated with the translucent clouds in Galactic arms is detected in absorption along every line of sight in all the ground state transitions. The continuum sources all exhibit broad water features in emission in the excited and ground state transitions. Strong absorption features associated with the source are also observed at all frequencies except 752 GHz. We model the background continuum and line emission to infer the optical depth of each translucent cloud along the lines of sight. We derive the column density of H_2O or H_2^(18)O for the lower energy level of each transition observed. The total column density of water in translucent clouds is usually about a few 10^(13) cm^(–2). We find that the abundance of water relative to hydrogen nuclei is 1 × 10^(–8) in agreement with models for oxygen chemistry in which high cosmic ray ionization rates are assumed. Relative to molecular hydrogen, the abundance of water is remarkably constant through the Galactic plane with X(H_2O) =5 × 10^(–8), which makes water a good traced of H_2 in translucent clouds. Observations of the excited transitions of H_2O enable us to constrain the abundance of water in excited levels to be at most 15%, implying that the excitation temperature, T_(ex), in the ground state transitions is below 10 K. Further analysis of the column densities derived from the two ortho ground state transitions indicates that T_(ex) ≃ 5 K and that the density n(H_2) in the translucent clouds is below 10^4 cm^(–3). We derive the water ortho-to-para ratio for each absorption feature along the line of sight and find that most of the clouds show ratios consistent with the value of 3 expected in thermodynamic equilibrium in the high-temperature limit. However, two clouds with large column densities exhibit a ratio that is significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains in the well-shielded, low-temperature regions of the clouds, or when they later become at least partially thermalized with the cold gas (~25 K) in those regions; evidently, they have not yet fully thermalized with the warmer (~50 K) translucent portions of the clouds.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Flagey, N.0000-0002-8763-1555
Goldsmith, P. F.0000-0002-6622-8396
Lis, D. C.0000-0002-0500-4700
Neufeld, D.0000-0001-8341-1646
Additional Information:© 2013 American Astronomical Society. Received 2012 August 25; accepted 2012 November 2; published 2012 December 7. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. This work was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, supported by NASA. M.G. and M.D.L. acknowledge the support from the Centre National de Recherche Spatiale (CNRS), and from ANR through the SCHISM project (ANR- 09-BLAN-231). J.R.G. is supported by a Ramόn y Cajal research contract and thanks the Spanish MICINN for funding support through grants AYA2009-07304 and CSD2009-00038. The Herschel spacecraft was designed, built, tested, and launched under a contract to ESA managed by the Herschel/Planck Project team by an industrial consortium under the overall responsibility of the prime contractor Thales Alenia Space (Cannes), and including Astrium (Friedrichshafen) responsible for the payload module and for system testing at spacecraft level, Thales Alenia Space (Turin) responsible for the service module, and Astrium (Toulouse) responsible for the telescope, with in excess of a hundred subcontractors. 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 United States. 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 Astronόmico 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, JPL, NHSC. HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS, and SPIRE consortia.
Funding AgencyGrant Number
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Agence Nationale pour la Recherche (ANR)ANR-09-BLAN-231
Ramόn y Cajal ProgrammeUNSPECIFIED
Ministerio de Ciencia e Innovación (MCINN)AYA2009-07304
Ministerio de Ciencia e Innovación (MCINN)CSD2009-00038
European Space Agency (ESA)UNSPECIFIED
Subject Keywords:astrochemistry; ISM: abundances; ISM: molecules; molecular processes; submillimeter: ISM
Issue or Number:1
Record Number:CaltechAUTHORS:20130205-082337964
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Official Citation:Water Absorption in Galactic Translucent Clouds: Conditions and History of the Gas Derived from Herschel/HIFI PRISMAS Observations N. Flagey et al. 2013 ApJ 762 11
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:36767
Deposited By: Ruth Sustaita
Deposited On:05 Feb 2013 17:17
Last Modified:09 Nov 2021 23:24

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