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Herschel HIFI observations of O_2 toward Orion: special conditions for shock enhanced emission

Chen, Jo-Hsin and Goldsmith, Paul F. and Viti, Serena and Snell, Ronald and Lis, Dariusz C. and Benz, Arnold and Bergin, Edwin and Black, John and Caselli, Paola and Encrenaz, Pierre and Falgarone, Edith and Goicoechea, Javier R. and Hjalmarson, Åke and Hollenbach, David and Kaufman, Michael and Melnick, Gary and Neufeld, David and Pagani, Laurent and van der Tak, Floris and van Dishoeck, Ewine F. and Yildiz, Umut A. (2014) Herschel HIFI observations of O_2 toward Orion: special conditions for shock enhanced emission. Astrophysical Journal, 793 (2). Art. No. 111. ISSN 0004-637X. doi:10.1088/0004-637X/793/2/111.

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We report observations of molecular oxygen (O_2) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O_2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km s^(-1) with line widths ~3 km s^(-3); however, the transition at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O_2 emission is ≃9" (6x10^(16) cm) in size, and is located close to the H_2 Peak 1 position (where vibrationally-excited H_2 emission peaks), and not at Peak A, 23" away. The peak O_2 column density is ≃1.1x10^(18)/cm^(-2). The line velocity is close to that of 621 GHz water maser emission found in this portion of the Orion Molecular Cloud, and having a shock with velocity vector lying nearly in the plane of the sky is consistent with producing maximum maser gain along the line-of-sight. The enhanced O_2 abundance compared to that generally found in dense interstellar clouds can be explained by passage of a low-velocity C-shock through a clump with preshock density 2x10^4/cm^(-3), if a reasonable flux of UV radiation is present. The postshock O_2 can explain the emission from the source if its line of sight dimension is ≃10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O_2 emission has not been detected in the cores of other massive star-forming molecular clouds.

Item Type:Article
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URLURL TypeDescription Paper
Goldsmith, Paul F.0000-0002-6622-8396
Lis, Dariusz C.0000-0002-0500-4700
Bergin, Edwin0000-0003-4179-6394
Caselli, Paola0000-0003-1481-7911
Neufeld, David0000-0001-8341-1646
van Dishoeck, Ewine F.0000-0001-7591-1907
Alternate Title:Herschel HIFI observations of O2 toward Orion: special conditions for shock enhanced emission
Additional Information:© 2014 American Astronomical Society. Received 2014 May 29; accepted 2014 August 6; published 2014 September 12. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States (NASA) under the leadership of SRON, Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France and the US. This work was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology. JRG thanks the Spanish MINECO for funding support under grants CSD2009-00038, AYA2009-07304, and AYA2012-32032. We thank Shiya Wang, Nathan Crockett, and the NASA Herschel Science Center Helpdesk for the help with the data reduction. We appreciate the help from Justin Neill with installing and using XCLASS. We thank Tzu-Cheng Peng for providing the CO map used in Figure 1. We thank the anonymous reviewer for a number of suggestions that improved the clarity of the paper.
Funding AgencyGrant Number
Ministerio de Economía y Competitividad (MINECO)CSD2009-00038
Ministerio de Economía y Competitividad (MINECO)AYA2009-07304
Ministerio de Economía y Competitividad (MINECO)AYA2012-32032
Subject Keywords: astrochemistry; ISM: molecules; shock waves
Issue or Number:2
Record Number:CaltechAUTHORS:20140825-105732969
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Official Citation:Herschel HIFI Observations of O2 toward Orion: Special Conditions for Shock Enhanced Emission Jo-Hsin Chen et al. 2014 ApJ 793 111
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48845
Deposited By: Ruth Sustaita
Deposited On:25 Aug 2014 18:36
Last Modified:10 Nov 2021 18:36

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