Water in star-forming regions with Herschel (WISH)

II. Evolution of 557 GHz 1₁₀–1₀₁ emission in low-mass protostars^⋆,⋆⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: kristensen@strw.leidenuniv.nl
² Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁴ Niels Bohr Institute and Centre for Star and Planet Formation, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø., Denmark
⁵ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁶ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
⁷ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁸ LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
¹² Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
¹³ CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
¹⁴ Joint ALMA Offices, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁵ Earth and Space Sciences, Chalmers University of Technology, 412 96 Gothenburg, Sweden
¹⁶ INAF – Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
¹⁷ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain
¹⁸ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
¹⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
²⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 24 September 2011
Accepted: 19 March 2012

Abstract

Context. Water is a key tracer of dynamics and chemistry in low-mass star-forming regions, but spectrally resolved observations have so far been limited in sensitivity and angular resolution, and only data from the brightest low-mass protostars have been published.

Aims. The first systematic survey of spectrally resolved water emission in 29 low-mass (L < 40   L_⊙) protostellar objects is presented. The sources cover a range of luminosities and evolutionary states. The aim is to characterise the line profiles to distinguish physical components in the beam and examine how water emission changes with protostellar evolution.

Methods. H₂O was observed in the ground-state 1₁₀–1₀₁ transition at 557 GHz (E_up/k_B ~ 60 K) as single-point observations with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel in 29 deeply embedded Class 0 and I low-mass protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our programme) are used to constrain the spectral energy distribution (SED) of each source. H₂O intensities are compared to inferred envelope properties, e.g., mass and density, outflow properties and CO 3–2 emission.

Results. H₂O emission is detected in all objects except one (TMC1A). The line profiles are complex and consist of several kinematic components tracing different physical regions in each system. In particular, the profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not in the quiescent envelope. Several sources show multiple shock components appearing in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in seven sources (six Class 0, one Class I) indicative of infalling envelopes, and regular P-Cygni profiles in four sources (three Class I, one Class 0) indicative of expanding envelopes. Molecular “bullets” moving at  ≳50 km s^-1 with respect to the source are detected in four Class 0 sources; three of these sources were not known to harbour bullets previously. In the outflow, the H₂O/CO abundance ratio as a function of velocity is nearly the same for all line wings, increasing from 10^-3 at low velocities (<5 km s^-1) to  ≳10^-1 at high velocities (>10 km s^-1). The water abundance in the outer cold envelope is low,  ≳10^-10. The different H₂O profile components show a clear evolutionary trend: in the younger Class 0 sources the emission is dominated by outflow components originating inside an infalling envelope. When large-scale infall diminishes during the Class I phase, the outflow weakens and H₂O emission all but disappears.