Multi-line detection of O2 toward rho Ophiuchi A
Journal article, 2012
Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O-2, and water, H2O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O-2 in the dense star-forming interstellar medium. Aims. Only toward rho OphA did Odin detect a very weak line of O-2 at 119 GHz in a beam of size 10 arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O-2 emission and pinpoint its origin. Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O-2 spectra toward selected positions in the rho Oph A core. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. Results. The N-J = 3(3)-1(2) line at 487.2 GHz is clearly detected toward all three observed positions in the rho Oph A core. In addition, an oversampled map of the 5(4)-3(4) transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O-2 emitting gas appears to vary quite substantially, with warm gas (greater than or similar to 50 K) being adjacent to a much colder region, of temperatures lower than 30 K. Conclusions. The exploited models predict that the O-2 column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O-2 column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O-2) = 3 to greater than or similar to 6 x 10(15) cm(-2). Beam-averaged O-2 abundances are about 5 x 10(-8) relative to H-2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire rho Oph A core. We speculate that one of the reasons for the generally very low detection rate of O-2 is the short period of time during which O-2 molecules are reasonably abundant in molecular clouds.
main cloud
interstellar hydrogen-peroxide
dust grains
c-i emission
ISM: abundances
satellite
oph cloud
ISM: individual objects: rho Oph
submillimeter observations
odin
photodissociation regions
dark-cloud
ISM: lines and bands
molecular
ISM: clouds
clouds
ISM: molecules
Author
René Liseau
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
P. F. Goldsmith
Jet Propulsion Laboratory, California Institute of Technology
B. Larsson
Stockholm University
L. Pagani
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
Per Bergman
Chalmers, Earth and Space Sciences, Onsala Space Observatory
J. Le Bourlot
LUTH - Laboratoire de l'Univers et de ses Theories
T. A. Bell
Centro de Astrobiologia (CAB)
A. O. Benz
Swiss Federal Institute of Technology in Zürich (ETH)
E. A. Bergin
University of Michigan
Per Bjerkeli
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
John H Black
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
S. Bruderer
Max Planck Society
Swiss Federal Institute of Technology in Zürich (ETH)
P. Caselli
University of Leeds
E. Caux
University of Toulouse
Jo-Hsin Chen
Jet Propulsion Laboratory, California Institute of Technology
M. de Luca
LUTH - Laboratoire de l'Univers et de ses Theories
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)
J. R. Goicoechea
Centro de Astrobiologia (CAB)
Åke Hjalmarson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
D. J. Hollenbach
SETI Institute
Kay Justtanont
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
M. J. Kaufman
San Jose State University
F. Le Petit
LUTH - Laboratoire de l'Univers et de ses Theories
D. Li
Chinese Academy of Sciences
California Institute of Technology (Caltech)
D. C. Lis
California Institute of Technology (Caltech)
G. J. Melnick
Harvard-Smithsonian Center for Astrophysics
Z. Nagy
University of Groningen
Netherlands Institute for Space Research (SRON)
Henrik Olofsson
Chalmers, Earth and Space Sciences, Onsala Space Observatory
G. Olofsson
Stockholm University
E. Roueff
LUTH - Laboratoire de l'Univers et de ses Theories
A. Sandqvist
Stockholm University
R. L. Snell
University of Massachusetts
F. F. S. van der Tak
Netherlands Institute for Space Research (SRON)
University of Groningen
E. F. van Dishoeck
Leiden University
Max Planck Society
C. Vastel
University of Toulouse
Centre national de la recherche scientifique (CNRS)
S. Viti
University College London (UCL)
U. A. Yildiz
Leiden University
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 541 A73Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201118575