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A Herschel/HIFI Legacy Survey of HF and H_2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry

Sonnentrucker, P. and Wolfire, M. and Neufeld, D. A. and Flagey, N. and Gerin, M. and Goldsmith, P. and Lis, D. and Monje, R. (2015) A Herschel/HIFI Legacy Survey of HF and H_2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry. Astrophysical Journal, 806 (1). Art. No. 49. ISSN 0004-637X. doi:10.1088/0004-637X/806/1/49.

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We combine Herschel observations for a total of 12 sources to construct the most uniform survey of HF and H_2O in our Galactic disk. Both molecules are detected in absorption along all sight lines. The high spectral resolution of the Heterodyne Instrument for the Far-infrared (HIFI) allows us to compare the HF and H_2O distributions in 47 diffuse cloud components sampling the disk. We find that the HF and H2O velocity distributions follow each other almost perfectly and establish that HF and H_2O probe the same gas-phase volume. Our observations corroborate theoretical predictions that HF is a sensitive tracer of H_2 in diffuse clouds, down to molecular fractions of only a few percent. Using HF to trace H_2 in our sample, we find that the N(H_2O)-to-N(HF) ratio shows a narrow distribution with a median value of 1.51. Our results further suggest that H_2O might be used as a tracer of H_2—within a factor of 2.5—in the diffuse interstellar medium (ISM). We show that the measured factor of ~2.5 variation around the median is driven by true local variations in the H_2O abundance relative to H_2 throughout the disk. The latter variability allows us to test our theoretical understanding of the chemistry of oxygen-bearing molecules in the diffuse gas. We show that both gas-phase and grain-surface chemistry are required to reproduce our H_2O observations. This survey thus confirms that grain surface reactions can play a significant role in the chemistry occurring in the diffuse ISM (n_H ≤ 1000 cm^(−3)).

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Wolfire, M.0000-0003-0030-9510
Neufeld, D. A.0000-0001-8341-1646
Flagey, N.0000-0002-8763-1555
Goldsmith, P.0000-0002-6622-8396
Lis, D.0000-0002-0500-4700
Additional Information:© 2015 The American Astronomical Society. Received 2014 December 15; accepted 2015 April 9; published 2015 June 5. We thank the referee for very useful comments that improved the manuscript significantly. We thank David Hollenbach for useful comments on grain surface chemistry. P.S. acknowledges support for this work from NASA through award NNN12AA01C issued by JPL/Caltech. M.G.W. was supported in part by NSF grant AST-1411827. M.G. thanks CNES and PCMI/INSU for support for the analysis of the Herschel data. This work was carried out in part at the Jet Propulsion Laboratory, which is operated for NASA by the California Institute of Technology. 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 Astronomico Nacional (IGN), Centro de Astrobiologa (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. Facilities: Herschel. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Programme National de Physique Chimie du Milieu Interstellaire (PCMI)UNSPECIFIED
Institut national des sciences de l'Univers (INSU)UNSPECIFIED
Subject Keywords:astrochemistry; cosmic rays; ISM: abundances; ISM: clouds; ISM: lines and bands; ISM: molecules
Issue or Number:1
Record Number:CaltechAUTHORS:20150720-100801032
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Official Citation:A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry P. Sonnentrucker et al. 2015 ApJ 806 49
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58939
Deposited On:24 Jul 2015 18:57
Last Modified:10 Nov 2021 22:12

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