This article has an erratum: [erratum]
Volume 572, December 2014
|Number of page(s)||49|
|Section||Interstellar and circumstellar matter|
|Published online||21 November 2014|
Water in star-forming regions with Herschel (WISH)
V. The physical conditions in low-mass protostellar outflows revealed by multi-transition water observations⋆,⋆⋆,⋆⋆⋆
1 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
4 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
5 Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
6 Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
7 Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
8 School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
9 Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33270 Floirac Cedex, France
10 CNRS, LAB, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33270 Floirac Cedex, France
11 Joint Astronomy Centre, 660 North Aohoku Place, University Park, Hilo, HI 96720, USA
12 Department of Physics and Astronomy, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada
13 NRC-Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
14 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
15 Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
16 SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
17 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
18 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received: 24 May 2014
Accepted: 19 September 2014
Context. Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance for the conditions expected in various parts of the protostar, particularly the outflow.
Aims. We constrain and quantify the physical conditions probed by water in the outflow-jet system for Class 0 and I sources.
Methods. We present velocity-resolved Herschel HIFI spectra of multiple water-transitions observed towards 29 nearby Class 0/I protostars as part of the WISH guaranteed time key programme. The lines are decomposed into different Gaussian components, with each component related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then use non-LTE radex models to constrain the excitation conditions present in the two outflow-related components.
Results. Water emission at the source position is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H2 densities of order 105 − 108 cm-3 and H2O column densities of order 1016 − 1018 cm-2. H2O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10−200 AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1−30 AU.
Conclusions. Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. The similarity of the physical conditions is in contrast to off-source determinations which show similar densities but lower column densities and larger filling factors. We propose that this is due to the differences in shock properties and geometry between these positions. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H2O intensity between Class 0 and I sources.
Key words: stars: formation / ISM: jets and outflows / ISM: molecules / stars: protostars
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendices are available in electronic form at http://www.aanda.org
Reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A21
© ESO, 2014
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