Multi-line detection of O2 toward ρ Ophiuchi A⋆
R. Liseau1, P. F. Goldsmith2, B. Larsson3, L. Pagani4, P. Bergman5, J. Le Bourlot6, T. A. Bell7, A. O. Benz8, E. A. Bergin9, P. Bjerkeli1, J. H. Black1, S. Bruderer8, 21, P. Caselli10, E. Caux11, J.-H. Chen2, M. de Luca6, P. Encrenaz4, E. Falgarone12, M. Gerin12, J. R. Goicoechea7, Å. Hjalmarson1, D. J. Hollenbach13, K. Justtanont1, M. J. Kaufman14, F. Le Petit6, D. Li15, 16, D. C. Lis16, G. J. Melnick17, Z. Nagy18, A. O. H. Olofsson5, G. Olofsson3, E. Roueff6, Aa. Sandqvist3, R. L. Snell19, F. F. S. van der Tak18, E. F. van Dishoeck20, 21, C. Vastel11, S. Viti22 and U. A. Yıldız20
1 Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
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
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 NJ = 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.
Key words: ISM: abundances / ISM: molecules / ISM: lines and bands / ISM: clouds / ISM: individual objects:ρOph A SM 1 / stars: formation
© ESO, 2012