Water in star-forming regions with Herschel (WISH)

III. Far-infrared cooling lines in low-mass young stellar objects^⋆

¹ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Kavli Institut for Astronomy and Astrophysics, Yi He Yuan Lu 5, HaiDian Qu, Peking University, 100871 Beijing, PR China
⁴ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁵ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
⁶ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁷ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁸ INAF – Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
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 Institute of Mathematics, Polish Academy of Sciences, ul. Śniadeckich 8, 00-956 Warszawa, Poland
¹² Institute of Mathematics, University of Wroclaw, pl. Grunwaldzki 2/4, 50-384 Wroclaw, Poland
¹³ Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
¹⁴ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
¹⁵ CNRS, LAB, UMR 5804, 33271 Floirac Cedex, France
¹⁶ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁷ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
¹⁸ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁹ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3, 28014 Madrid, Spain
²⁰ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
²¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: karska@mpe.mpg.de

Received: 16 July 2012
Accepted: 16 January 2013

Abstract

Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope.

Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages.

Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H₂O, OH, and atomic lines. For four targets the spectra span the entire 55–200 μm region. The PACS field-of-view covers ~47″ with the resolution of 9.4″.

Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H₂O 8₁₈–7₀₇ 63.3 μm line (E_u/k_B = 1071 K) is detected in 7 sources. CO transitions from J = 14−13 up to J = 49 − 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H₂O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on ≥10 000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H₂O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O i] fluxes. In contrast, [O i] and OH often peak together at the central position.

Conclusions. The PACS data probe at least two physical components. The H₂O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H₂O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24–23 line and the CO 14–13 line. [O i] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H₂O and CO, with the fraction contributed by [O i] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state.