HERSCHEL SEARCH FOR O-2 TOWARD THE ORION BAR *
Journal article, 2012
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 HII region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O-2 N-J = 3(3)-1(2) 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 3 sigma upper limits established here translate to a total line-of-sight O-2 column density <1.5 x 10(16) cm(-2) for an emitting region whose temperature is between 30 K and 250 K, or <1 x 10(16) cm(-2) if the O-2 emitting region is primarily at a temperature of less than or similar to 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 x 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 AV 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.
water ice
emission
submillimeter: ISM
chemical-models
molecular clouds
region
interstellar
millimeter
ISM: abundances
thermal balance
astrochemistry
photodesorption
ISM: individual objects (Orion)
chloronium
ISM: molecules
submillimeter observations
Author
G. J. Melnick
Harvard-Smithsonian Center for Astrophysics
V. Tolls
Harvard-Smithsonian Center for Astrophysics
P. F. Goldsmith
California Institute of Technology (Caltech)
M. J. Kaufman
San Jose State University
D. J. Hollenbach
SETI Institute
John H Black
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
P. Encrenaz
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
E. Falgarone
Centre national de la recherche scientifique (CNRS)
M. Gerin
Centre national de la recherche scientifique (CNRS)
Åke Hjalmarson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
D. Li
Chinese Academy of Sciences
D. C. Lis
California Institute of Technology (Caltech)
René Liseau
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
D. A. Neufeld
Johns Hopkins University
L. Pagani
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
R. L. Snell
University of Massachusetts
F. F. S. van der Tak
University of Groningen
E. F. van Dishoeck
Max Planck Society
Leiden University
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 752 1 art. no. 26 (pp. 1-9) 26Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
Infrastructure
Onsala Space Observatory
DOI
10.1088/0004-637X/752/1/26