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Title: HERSCHEL SEARCH FOR O{sub 2} TOWARD THE ORION BAR

Abstract

We report the results of a search for molecular oxygen (O{sub 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{sub 2} N{sub J} = 3{sub 3}-1{sub 2} transition at 487 GHz and the 5{sub 4}-3{sub 4} transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3{sigma} upper limits established here translate to a total line-of-sight O{sub 2} column density <1.5 Multiplication-Sign 10{sup 16} cm{sup -2} for an emitting region whose temperature is between 30 K and 250 K, or <1 Multiplication-Sign 10{sup 16} cm{sup -2} if the O{sub 2} emitting region is primarily at a temperature of {approx}<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{sub 2} column density is less than 4 Multiplication-Sign 10{sup 15} cm{sup -2}, a value that is below, and possiblymore » well below, model predictions for gas with a density of 10{sup 4}-10{sup 5} cm{sup -3} exposed to a far-ultraviolet flux 10{sup 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{sub V} of the Bar is less than required for O{sub 2} to reach peak abundance; (3) the O{sub 2} emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O{sub 2} reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.« less

Authors:
;  [1];  [2];  [3];  [4]; ; ;  [5]; ;  [6]; ;  [7];  [8];  [9];  [10];  [11];  [12];  [13]
  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 66, Cambridge, MA 02138 (United States)
  2. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States)
  3. Department of Physics and Astronomy, San Jose State University, San Jose, CA 95192 (United States)
  4. SETI Institute, Mountain View, CA 94043 (United States)
  5. Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala (Sweden)
  6. LERMA and UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris (France)
  7. LRA/LERMA, CNRS, UMR8112, Observatoire de Paris and Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris Cedex 05 (France)
  8. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012 (China)
  9. California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125 (United States)
  10. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States)
  11. Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
  12. SRON Netherlands Institute for Space Research, P.O. Box 800, 9700 AV, and Kapteyn Astronomical Institute, University of Groningen, Groningen (Netherlands)
  13. Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden (Netherlands)
Publication Date:
OSTI Identifier:
22037091
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 752; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; ADSORPTION; ASTRONOMY; ASTROPHYSICS; ATOMS; COSMOCHEMISTRY; DENSITY; DISSOCIATION; EMISSION; FAR INFRARED RADIATION; FAR ULTRAVIOLET RADIATION; GHZ RANGE; HYDROGEN; MOLECULES; OXYGEN; PHOTOLYSIS; STARS

Citation Formats

Melnick, Gary J., Tolls, Volker, Goldsmith, Paul F., Kaufman, Michael J., Hollenbach, David J., Black, John H., Hjalmarson, Ake, Liseau, Rene, Encrenaz, Pierre, Pagani, Laurent, Falgarone, Edith, Gerin, Maryvonne, Li, Di, Lis, Dariusz C., Neufeld, David A., Snell, Ronald L., Van der Tak, Floris, and Van Dishoeck, Ewine F. HERSCHEL SEARCH FOR O{sub 2} TOWARD THE ORION BAR. United States: N. p., 2012. Web. doi:10.1088/0004-637X/752/1/26.
Melnick, Gary J., Tolls, Volker, Goldsmith, Paul F., Kaufman, Michael J., Hollenbach, David J., Black, John H., Hjalmarson, Ake, Liseau, Rene, Encrenaz, Pierre, Pagani, Laurent, Falgarone, Edith, Gerin, Maryvonne, Li, Di, Lis, Dariusz C., Neufeld, David A., Snell, Ronald L., Van der Tak, Floris, & Van Dishoeck, Ewine F. HERSCHEL SEARCH FOR O{sub 2} TOWARD THE ORION BAR. United States. doi:10.1088/0004-637X/752/1/26.
Melnick, Gary J., Tolls, Volker, Goldsmith, Paul F., Kaufman, Michael J., Hollenbach, David J., Black, John H., Hjalmarson, Ake, Liseau, Rene, Encrenaz, Pierre, Pagani, Laurent, Falgarone, Edith, Gerin, Maryvonne, Li, Di, Lis, Dariusz C., Neufeld, David A., Snell, Ronald L., Van der Tak, Floris, and Van Dishoeck, Ewine F. Sun . "HERSCHEL SEARCH FOR O{sub 2} TOWARD THE ORION BAR". United States. doi:10.1088/0004-637X/752/1/26.
@article{osti_22037091,
title = {HERSCHEL SEARCH FOR O{sub 2} TOWARD THE ORION BAR},
author = {Melnick, Gary J. and Tolls, Volker and Goldsmith, Paul F. and Kaufman, Michael J. and Hollenbach, David J. and Black, John H. and Hjalmarson, Ake and Liseau, Rene and Encrenaz, Pierre and Pagani, Laurent and Falgarone, Edith and Gerin, Maryvonne and Li, Di and Lis, Dariusz C. and Neufeld, David A. and Snell, Ronald L. and Van der Tak, Floris and Van Dishoeck, Ewine F.},
abstractNote = {We report the results of a search for molecular oxygen (O{sub 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{sub 2} N{sub J} = 3{sub 3}-1{sub 2} transition at 487 GHz and the 5{sub 4}-3{sub 4} transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3{sigma} upper limits established here translate to a total line-of-sight O{sub 2} column density <1.5 Multiplication-Sign 10{sup 16} cm{sup -2} for an emitting region whose temperature is between 30 K and 250 K, or <1 Multiplication-Sign 10{sup 16} cm{sup -2} if the O{sub 2} emitting region is primarily at a temperature of {approx}<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{sub 2} column density is less than 4 Multiplication-Sign 10{sup 15} cm{sup -2}, a value that is below, and possibly well below, model predictions for gas with a density of 10{sup 4}-10{sup 5} cm{sup -3} exposed to a far-ultraviolet flux 10{sup 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{sub V} of the Bar is less than required for O{sub 2} to reach peak abundance; (3) the O{sub 2} emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O{sub 2} reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.},
doi = {10.1088/0004-637X/752/1/26},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 752,
place = {United States},
year = {2012},
month = {6}
}