The Herschel/HIFI spectral survey of OMC-2 FIR 4 (CHESS)

An overview of the 480 to 1902 GHz range^⋆

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: mkama@strw.leidenuniv.nl
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³ UJF–Grenoble 1/CNRS–INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble, France
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Mailbox 79, Po Box 9010, 6525 AJ, Nijmegen, The Netherlands
⁵ Observatorio Astronómico Nacional, PO Box 112, 28803 Alcalá de Henares, Madrid, Spain
⁶ Université de Toulouse, UPS–OMP, IRAP, 31400 Toulouse, France
⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 17 April 2012
Accepted: 2 May 2013

Abstract

Context. Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened up the terahertz regime to such surveys, giving access to the fundamental transitions of many hydrides and to the high-energy transitions of many other species.

Aims. A comparative analysis of the chemical inventories and physical processes and properties of protostars of various masses and evolutionary states is the goal of the Herschel  CHEmical Surveys of Star forming regions (CHESS) key program. This paper focusses on the intermediate-mass protostar, OMC-2 FIR 4.

Methods. We obtained a spectrum of OMC-2 FIR 4 in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel  and carried out the reduction, line identification, and a broad analysis of the line profile components, excitation, and cooling.

Results. We detect 719 spectral lines from 40 species and isotopologs. The line flux is dominated by CO, H₂O, and CH₃OH. The line profiles are complex and vary with species and upper level energy, but clearly contain signatures from quiescent gas, a broad component likely due to an outflow, and a foreground cloud.

Conclusions. We find abundant evidence for warm, dense gas, as well as for an outflow in the field of view. Line flux represents 2% of the 7  L_⊙ luminosity detected with HIFI in the 480 to 1250 GHz range. Of the total line flux, 60% is from CO, 13% from H₂O and 9% from CH₃OH. A comparison with similar HIFI spectra of other sources is set to provide much new insight into star formation regions, a case in point being a difference of two orders of magnitude in the relative contribution of sulphur oxides to the line cooling of Orion KL and OMC-2 FIR 4.