Letter to the Editor

Water cooling of shocks in protostellar outflows^*

Herschel-PACS map of L1157

¹ INAF - Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: nisini@oa-roma.inaf.it
² INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
³ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁶ IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Max Planck Institut for Extraterestrische Physik, Garching, Germany
⁹ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
¹⁰ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹¹ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹² California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹³ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁴ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain
¹⁵ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
¹⁶ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁸ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
¹⁹ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
²⁰ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²¹ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²² Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
²³ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
²⁴ Department of Physics and Astronomy, San Jose State University, One Washington Square, San Jose, CA 95192, USA
²⁵ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
²⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁸ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
²⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
³⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
³² Institute Laboratoire d'Etudes du Rayonnement et de la Matire en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
³³ CNRS/INSU, UMR 5804, B.P. 89, 33271 Floirac cedex, France
³⁴ Institute Institut de Radioastronomie Millimetrique, IRAM, 300 rue de la Piscine, 38406 St Martin d'Heres, France
³⁵ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³⁶ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
³⁷ ALMA

Received: 31 March 2010
Accepted: 23 April 2010

Abstract

Context. The far-IR/sub-mm spectral mapping facility provided by the Herschel-PACS and HIFI instruments has made it possible to obtain, for the first time, images of H₂O emission with a spatial resolution comparable to ground based mm/sub-mm observations.

Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H₂O 179 μm transition obtained toward the young outflow L1157.

Methods. The 179 μm map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the H₂O abundance and total cooling.

Results. Strong H₂O peaks are localized on both shocked emission knots and the central source position. The H₂O 179 μm emission is spatially correlated with emission from H₂ rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH₃. A strong H₂O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 μm intensity and its intensity ratio to the H₂O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H₂O  abundance of the order of 10^-4. This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total H₂O cooling is ~10^-1 , about 40% of the cooling due to H₂ and 23% of the total energy released in shocks along the L1157 outflow.