Deep observations of O₂ toward a low-mass protostar with Herschel-HIFI^{⋆,⋆⋆,⋆⋆⋆}

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: yildiz@strw.leidenuniv.nl
² S.N. Bose National Centre for Basic Sciences, 700098 Kolkata, Salt Lake, India
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁴ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Department of Astronomy, LGRT 619, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁷ Dept. of Earth & Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁸ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁹ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹⁰ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹² Departments of Chemistry, Astronomy, and Physics, The University of Virginia, Charlottesville, Virginia, USA
¹³ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
¹⁴ LRA/LERMA, CNRS, UMR 8112, Observatoire de Paris & École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 5, France

Received: 22 May 2013
Accepted: 26 July 2013

Abstract

Context. According to traditional gas-phase chemical models, O₂ should be abundant in molecular clouds, but until recently, attempts to detect interstellar O₂ line emission with ground- and space-based observatories have failed.

Aims. Following the multi-line detections of O₂ with low abundances in the Orion and ρ Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O₂ abundance near a solar-mass protostar.

Methods. Observations of molecular oxygen, O₂, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models.

Results. The deep HIFI spectrum fails to show O₂ at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O₂/H₂ at ≤6 × 10^-9 (3σ). The O₂/CO abundance ratio is less than 0.005. However, a tentative (4.5σ) detection of O₂ is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km s^-1 relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7–1 × 10⁶ years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H₂O, to avoid overproduction of O₂ in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule if hydrogenation of NO on the grains to more complex molecules such as NH₂OH, found in recent laboratory experiments, is included. The tentative detection of O₂ in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates.

Conclusions. The low O₂ abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O₂.