Multi-line detection of O₂ toward ρ Ophiuchi A^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: rene.liseau@chalmers.se
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109, USA
³ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁴ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁵ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁶ Observatoire de Paris, LUTH, Paris, France
⁷ Centro de Astrobiología, CSICINTA, 28850 Madrid, Spain
⁸ Institute of Astronomy, ETH-Zurich, Zurich, Switzerland
⁹ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor MI 48109, USA
¹⁰ School of Physics and Astronomy, University of Leeds, Leeds, UK
¹¹ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France & CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹² LRA/LERMA, CNRS, UMR8112, Observatoire de Paris & École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
¹³ SETI Institute, Mountain View CA 94043, USA
¹⁴ Department of Physics and Astronomy, San José State University, San Jose CA 95192, USA
¹⁵ National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, 100012 Beijing, PR China
¹⁶ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, 1200 E. California Boulevard, Pasadena CA 91125, USA
¹⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 66, Cambridge MA 02138, USA
¹⁸ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, and Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
¹⁹ Department of Astronomy, University of Massachusetts, Amherst MA 01003, USA
²⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²¹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
²² Department of Physics and Astronomy, University College London, London, UK

Received: 4 December 2011
Accepted: 7 March 2012

Abstract

Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O₂, 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 O₂ in the dense star-forming interstellar medium.

Aims. Only toward ρ   Oph   A did Odin detect a very weak line of O₂ 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 O₂ emission and pinpoint its origin.

Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O₂ spectra toward selected positions in the ρ   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 = 3₃ − 1₂ line at 487.2 GHz is clearly detected toward all three observed positions in the ρ   Oph   A   core. In addition, an oversampled map of the 5₄−3₄ 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 O₂ emitting gas appears to vary quite substantially, with warm gas ( ≳ 50 K) being adjacent to a much colder region, of temperatures lower than 30 K.

Conclusions. The exploited models predict that the O₂ 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 O₂ column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O₂) = 3 to  ≳ 6  ×  10¹⁵ cm^-2. Beam-averaged O₂ abundances are about 5 × 10^-8 relative to H₂. 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 ρ   Oph   A   core. We speculate that one of the reasons for the generally very low detection rate of O₂ is the short period of time during which O₂ molecules are reasonably abundant in molecular clouds.