High-angular resolution observations towards OMC-2 FIR 4: Dissecting an intermediate-mass protocluster^⋆

¹ UJF – Grenoble 1/CNRS–INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: ana.sepulcre@obs.ujf-grenoble.fr
² Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50124 Firenze, Italy
³ Astronomical Institute Anton Pannekoek, University of Amsterdam, PO BOX 94249 1090 GE Amsterdam, The Netherlands
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, 6525 AJ Nijmegen, The Netherlands
⁵ Université de Toulouse, UPS–OMP, IRAP, 31028 Toulouse Cedex 4, France
⁶ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁷ Observatorio Astronómico Nacional, PO Box 112, 28803 Alcalá de Henares, Madrid, Spain
⁸ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 106 Taipei, Taiwan
⁹ Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ IRAM, 300 rue de la piscine, 38406 Saint-Martin d’Hères, France
¹¹ Nobeyama Radio Observatory, 462-2 Nobeyama, Minamimaki, Minamisaku, 384-1305 Nagano, Japan

Received: 13 December 2012
Accepted: 16 April 2013

Abstract

Context. Intermediate-mass stars are an important ingredient of our Galaxy and a key to understanding how high- and low-mass stars form in clusters. One of the closest known young intermediate-mass protoclusters is OMC-2 FIR 4, which is located at a distance of 420 pc in Orion. This region is one of the few where the complete 500–2000 GHz spectrum has been observed with the heterodyne spectrometer HIFI on board the Herschel satellite, and unbiased spectral surveys at 0.8, 1, 2, and 3 mm have been obtained with the JCMT and IRAM 30-m telescopes.

Aims. We aim to disentangle the core multiplicity, to investigate the morphology of this region in order to study the formation of a low- and intermediate-mass protostar cluster, and to aid in interpretation of the single-dish line profiles already in our hands.

Methods. We used the IRAM Plateau de Bure Interferometer to image OMC-2 FIR 4 in the 2-mm continuum emission, as well as in DCO⁺(2–1), DCN(2–1), C³⁴S(3–2), and several CH₃OH lines. In addition, we analysed observations of the NH₃(1, 1) and (2, 2) inversion transitions that used the Very Large Array of the NRAO. The resulting maps have an angular resolution that allows us to resolve structures of 5″, which is equivalent to ~2000 AU.

Results. Our observations reveal three spatially resolved sources within OMC-2 FIR 4, of one or several solar masses each, with hints of further unresolved substructure within them. Two of these sources have elongated shapes and are associated with dust continuum emission peaks, thus likely containing at least one molecular core each. One of them also displays radio continuum emission, which may be attributed to a young B3–B4 star that dominates the overall luminosity output of the region. The third identified source displays a DCO⁺(2–1) emission peak and weak dust continuum emission. Its higher abundance of DCO⁺ relative to the other two regions suggests a lower temperature, hence its possible association with either a younger low-mass protostar or a starless core. It may alternatively be part of the colder envelope of OMC-2 FIR 4.

Conclusions. Our interferometric observations show the complexity of the intermediate-mass protocluster OMC-2 FIR 4, where multiple cores, chemical differentiation, and an ionised region all coexist within an area of only 10 000 AU.