Water in star-forming regions with Herschel (WISH) III. Far-infrared cooling lines in low-mass young stellar objects
Artikel i vetenskaplig tidskrift, 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.

ISM: jets and outflows

stars: protostars

cologne database

molecular clouds

outflow l1157

thermal balance

ngc 1333

physical

molecular processes

submillimeter continuum observations

observations

infrared: ISM

j co

astrochemistry

structure

interstellar shocks

protostellar shock l1157-b1

Författare

A. Karska

Universiteit Leiden

Max Planck-institutet

G. J. Herczeg

Max Planck-institutet

Peking University

E. F. van Dishoeck

Universiteit Leiden

Max Planck-institutet

S. F. Wampfler

Eidgenössische Technische Hochschule Zürich (ETH)

Köbenhavns Universitet

L. Kristensen

Universiteit Leiden

J. R. Goicoechea

CSIC-INTA - Centro de Astrobiologia (CAB)

R. Visser

University of Michigan

B. Nisini

Osservatorio Astronomico di Roma

I. San Jose-Garcia

Universiteit Leiden

S. Bruderer

Max Planck-institutet

P. Sniady

University of Wroclaw

Institute of Mathematics of the Polish Academy of Sciences

S. Doty

Denison University

D. Fedele

Max Planck-institutet

U. A. Yildiz

Universiteit Leiden

A. O. Benz

Eidgenössische Technische Hochschule Zürich (ETH)

E. A. Bergin

University of Michigan

P. Caselli

University of Leeds

Osservatorio Astrofisico di Arcetri

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

Universite de Bordeaux

M. R. Hogerheijde

Universiteit Leiden

D. Johnstone

National Research Council Canada

University of Victoria

J. K. Jorgensen

Köbenhavns Universitet

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

M. Tafalla

Observatorio Astronomico Nacional

F. F. S. van der Tak

Rijksuniversiteit Groningen

Netherlands Institute for Space Research (SRON)

F. Wyrowski

Max Planck-institutet

Astronomy and Astrophysics

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

Vol. 552 A141

Ämneskategorier

Astronomi, astrofysik och kosmologi

DOI

10.1051/0004-6361/201220028