Water in star-forming regions with Herschel (WISH) III. Far-infrared cooling lines in low-mass young stellar objects
Journal article, 2013

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 mu m region. The PACS field-of-view covers similar to 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 8(18)-7(07) 63.3 mu m 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 similar to 350 K and similar to 700 K. H2O has typical excitation temperatures of similar to 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.

j co

thermal balance


ISM: jets and outflows


outflow l1157

submillimeter continuum observations

interstellar shocks

stars: protostars


ngc 1333

molecular processes

molecular clouds


protostellar shock l1157-b1

cologne database

infrared: ISM


A. Karska

Leiden University

Max Planck Society

G. J. Herczeg

Max Planck Society

Beijing University of Technology

E. F. van Dishoeck

Max Planck Society

Leiden University

S. F. Wampfler

Swiss Federal Institute of Technology in Zürich (ETH)

University of Copenhagen

L. Kristensen

Leiden University

J. R. Goicoechea

Centro de Astrobiologia (CAB)

R. Visser

University of Michigan

B. Nisini

Osservatorio Astronomico di Roma

I. San Jose-Garcia

Leiden University

S. Bruderer

Max Planck Society

P. Sniady

University of Wrocław

Polish Academy of Sciences

S. Doty

Denison University

D. Fedele

Max Planck Society

U. A. Yildiz

Leiden University

A. O. Benz

Swiss Federal Institute of Technology in Zürich (ETH)

E. A. Bergin

University of Michigan

P. Caselli

Arcetri Astrophysical Observatory

University of Leeds

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

University of Bordeaux

M. R. Hogerheijde

Leiden University

D. Johnstone

National Research Council Canada

University of Victoria

J. K. Jorgensen

University of Copenhagen

René Liseau

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

M. Tafalla

Spanish National Observatory (OAN)

F. F. S. van der Tak

Netherlands Institute for Space Research (SRON)

University of Groningen

F. Wyrowski

Max Planck Society

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 552 A141

Subject Categories

Astronomy, Astrophysics and Cosmology



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