Letter to the Editor

Origin of the hot gas in low-mass protostars ^*,**

Herschel-PACS spectroscopy of HH 46

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: kristensen@strw.leidenuniv.nl
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁵ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
⁶ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
⁷ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3. 28014, Madrid, Spain
⁸ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁹ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux; CNRS/INSU, UMR 5804, Floirac, France
¹⁰ INAF – Instituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹² Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹³ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹⁴ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁶ Centro de Astrobiología, Departamento de Astrofísica, CSIC-INTA, Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain
¹⁷ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁹ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
²⁰ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
²¹ Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain
²² INAF – Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
²³ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²⁴ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²⁵ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
²⁶ Department of Physics and Astronomy, San Jose State University, One Washington Square, San Jose, CA 95192, USA
²⁷ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²⁸ California Institute of Technology, Cahill Center for Astronomy and Astrophysics, MS 301-17, Pasadena, CA 91125, USA
²⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³¹ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
³² Instituto de RadioAstronomía Milimétrica, Avenida Divina Pastora, 7, Núcleo Central E 18012 Granada, Spain
³³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
³⁴ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³⁵ California Institute of Technology, 1200 E. California Bl., MC 100-22, Pasadena, CA 91125, USA
³⁶ Experimental Physics Dept., National University of Ireland Maynooth, Co. Kildare. Ireland

Received: 31 March 2010
Accepted: 10 May 2010

Abstract

Aims. “Water In Star-forming regions with Herschel” (WISH) is a Herschel key programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species.

Methods. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H₂O, CO, OH, [O i], and [C ii] lines located between 63 and 186 μm. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible.

Results. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H₂O emission is concentrated in the outflow. In addition, [O i] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km s^-1 in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H₂O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [O i] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 × 10^-2 , with 60–80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.