Herschel Measurements of Molecular Oxygen in Orion

Creators: Goldsmith, Paul F.; Lis, Dariusz C.

Abstract

We report observations of three rotational transitions of molecular oxygen (O_2) in emission from the H_2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory, having velocities of 11 km s^(–1) to 12 km s^(–1) and widths of 3 km s^(–1). The beam-averaged column density is N(O_2) = 6.5 × 10^(16) cm^(–2), and assuming that the source has an equal beam-filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O_2 relative to H_2 is (0.3-7.3) × 10^(–6). The unusual velocity suggests an association with a ~5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 M_⊙ and the dust temperature is ≥150 K. Our preferred explanation of the enhanced O_2 abundance is that dust grains in this region are sufficiently warm (T ≥ 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O_2. For this small source, the line ratios require a temperature ≥180 K. The inferred O_2 column density ≃5 × 10^(18) cm^(–2) can be produced in Peak A, having N(H_2) ≃4 × 10^(24) cm^(–2). An alternative mechanism is a low-velocity (10-15 km s^(–1)) C-shock, which can produce N(O_2) up to 10^(17) cm^(–2).

Additional Information

© 2011 American Astronomical Society. Received 2011 May 8; accepted 2011 June 10; published 2011 August 8. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We are indebted to the many people who worked so hard and for so long to make the Herschel mission and the HIFI instrument a success. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the US under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands with major contributions from Germany,France, and the US Consortium members are Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronόmico Nacional (IGN), Centro de Astrobiología (CSIC-INTA); Sweden: Chalmers University of Technology-MC2, RSS, & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University-Stockholm Observatory; Switzerland: ETH Zürich, FHNW; USA: Caltech, JPL, NHSC. The O2 excitation calculations were carried out using the RADEX code (Van der Tak et al. 2007). We appreciate the effort that went into making critical spectroscopic data available through the Jet Propulsion Laboratory Molecular Spectroscopy Data Base (http://spec.jpl.nasa.gov/), the Cologne Database for Molecular Spectroscopy, (http://www.astro.uni-oeln.de/cdms/ and Müller et al. 2001) and the Leiden Atomic and Molecular Database (http://www.strw.leidenuniv.nl/∼moldata/ and Schöier et al. 2005). We thank Holger Müller for helpful discussions about molecular spectroscopy. Colin Borys of the NASA Herschel Science Center gave us valuable assistance in unraveling the pointing offsets of the two HIFI polarization beams. We thank Nathaniel Cuningham and John Bally for sending us the FITS image used to make Figure 2. We appreciate the input from John Pearson and Harshal Gupta in terms of useful discussions about molecular structure and astrophysics. Volker Tolls provided valuable information about the WBS noise bandwidth and noise after combining WBS spectral channels. We have benefited from discussions with D. Quan about grain warmup and its impact on molecular cloud chemistry. The Caltech Submillimeter Observatory is supported by the NSF under award AST-0540882. This work was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We thank K. Öberg for helpful information on desorption of molecules from grains, and the referee for a number of valuable suggestions.

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