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.

ISM: lines and bands

ISM: abundances

submillimeter observations

ISM: individual objects: rho Oph

dust grains

clouds

ISM: molecules

molecular

dark-cloud

odin

satellite

oph cloud

c-i emission

ISM: clouds

interstellar hydrogen-peroxide

main cloud

photodissociation regions

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

CSIC-INTA - Centro de Astrobiologia (CAB)

A. O. Benz

Swiss Federal Institute of Technology in Zürich (ETH)

E. A. Bergin

University Michigan Ann Arbor

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 Institute

Swiss Federal Institute of Technology in Zürich (ETH)

P. Caselli

University of Leeds

E. Caux

Universite de 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

CNRS Centre National de la Recherche Scientifique

M. Gerin

CNRS Centre National de la Recherche Scientifique

J. R. Goicoechea

CSIC-INTA - 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

CalTech

National Astronomical Observatories Chinese Academy of Sciences

D. C. Lis

CalTech

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

Z. Nagy

University of Groningen, Kapteyn Astronomical Institute

SRON Netherlands Institute for Space Research

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

University of Groningen, Kapteyn Astronomical Institute

SRON Netherlands Institute for Space Research

E. F. van Dishoeck

Max Planck Institute

Leiden University

C. Vastel

CNRS Centre National de la Recherche Scientifique

Universite de Toulouse

S. Viti

University College London (UCL)

U. A. Yildiz

Leiden University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 541 A73

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/201118575