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Herschel Search for O_2 toward the Orion Bar

Melnick, Gary J. and Tolls, Volker and Goldsmith, Paul F. and Kaufman, Michael J. and Hollenbach, David J. and Black, John H. and Encrenaz, Pierre and Falgarone, Edith and Gerin, Maryvonne and Hjalmarson, Åke and Li, Di and Lis, Dariusz C. and Liseau, René and Neufeld, David A. and Pagani, Laurent and Snell, Ronald L. and van der Tak, Floris and van Dishoeck, Ewine F. (2012) Herschel Search for O_2 toward the Orion Bar. Astrophysical Journal, 752 (1). Art. No. 26. ISSN 0004-637X.

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We report the results of a search for molecular oxygen (O_2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the H II region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O_2 NJ = 33-12 transition at 487 GHz and the 5_(4)-3_(4) transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3σ upper limits established here translate to a total line-of-sight O2 column density <1.5 × 10^(16) cm^(–2) for an emitting region whose temperature is between 30 K and 250 K, or <1 × 10^(16) cm^(–2) if the O_2 emitting region is primarily at a temperature of ≲100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O_2 column density is less than 4 × 10^(15) cm^(–2), a value that is below, and possibly well below, model predictions for gas with a density of 10^(4)-10^(5) cm^(–3) exposed to a far-ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on A V of the Bar is less than required for O_2 to reach peak abundance; (3) the O_2 emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O_2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.

Item Type:Article
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URLURL TypeDescription
Goldsmith, Paul F.0000-0002-6622-8396
Lis, Dariusz C.0000-0002-0500-4700
Neufeld, David A.0000-0001-8341-1646
van Dishoeck, Ewine F.0000-0001-7591-1907
Additional Information:© 2012 American Astronomical Society. Received 2012 February 14; accepted 2012 April 9; published 2012 May 23. 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 Astronómico Nacional (IGN), Centro de Astrobiologá (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. We also acknowledge the effort that went into making critical spectroscopic data available through the Jet Propulsion Laboratory Molecular Spectroscopy Data Base (, the Cologne Database for Molecular Spectroscopy ( and Müller et al. 2005), and the Leiden Atomic and Molecular Database (∼moldata/ and Schöier et al. 2005). Finally, it is a pleasure to acknowledge useful discussions with Dr. Edwin Bergin. Support for this work was provided by NASA through an award issued by JPL/Caltech. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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Subject Keywords:astrochemistry; ISM: abundances; ISM: individual objects (Orion); ISM: molecules; submillimeter: ISM
Issue or Number:1
Record Number:CaltechAUTHORS:20120713-130439532
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Official Citation:Herschel Search for O2 toward the Orion Bar Gary J. Melnick et al. 2012 ApJ 752 26
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:32429
Deposited By: Jason Perez
Deposited On:13 Jul 2012 21:44
Last Modified:09 Mar 2020 13:19

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