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HERSCHEL SEARCH FOR O-2 TOWARD THE ORION BAR *

G. J. Melnick ; V. Tolls ; P. F. Goldsmith ; M. J. Kaufman ; D. J. Hollenbach ; John H. Black (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; P. Encrenaz ; E. Falgarone ; M. Gerin ; Åke Hjalmarson (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; D. Li ; D. C. Lis ; René Liseau (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; D. A. Neufeld ; L. Pagani ; R. L. Snell ; F. van der Tak ; E. F. van Dishoeck
Astrophysical Journal (0004-637X). Vol. 752 (2012), 1, p. art. no. 26 (pp. 1-9).
[Artikel, refereegranskad vetenskaplig]

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.

Nyckelord: astrochemistry, ISM: abundances, ISM: individual objects (Orion), ISM: molecules, submillimeter: ISM, submillimeter observations, molecular clouds, water ice, interstellar, chloronium, thermal balance, chemical-models, photodesorption, emission, millimeter, region


* Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.



Denna post skapades 2012-07-16. Senast ändrad 2014-09-02.
CPL Pubid: 160495

 

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