HERSCHEL SEARCH FOR O-2 TOWARD THE ORION BAR *
Artikel i vetenskaplig tidskrift, 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.

ISM: abundances

chemical-models

interstellar

millimeter

molecular clouds

submillimeter observations

emission

water ice

photodesorption

region

thermal balance

submillimeter: ISM

ISM: individual objects (Orion)

chloronium

ISM: molecules

astrochemistry

Författare

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

V. Tolls

Harvard-Smithsonian Center for Astrophysics

P. F. Goldsmith

Jet Propulsion Laboratory, California Institute of Technology

M. J. Kaufman

San Jose State University

D. J. Hollenbach

SETI Institute

John H Black

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Encrenaz

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

E. Falgarone

CNRS Centre National de la Recherche Scientifique

M. Gerin

CNRS Centre National de la Recherche Scientifique

Åke Hjalmarson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

D. Li

National Astronomical Observatories Chinese Academy of Sciences

D. C. Lis

CalTech

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

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

Rijksuniversiteit Groningen

E. F. van Dishoeck

Universiteit Leiden

Max Planck-institutet

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 752 art. no. 26 (pp. 1-9) 26

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

Infrastruktur

Onsala rymdobservatorium

DOI

10.1088/0004-637X/752/1/26