Water in star-forming regions with Herschel (WISH)
Artikel i vetenskaplig tidskrift, 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 H-2 densities of order 10(5) -10(8) cm(-3) and H2O column densities of order 10(16) -10(18) 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.

ISM: molecules

stars: protostars

ISM: jets and outflows

stars: formation


J. C. Mottram

Universiteit Leiden

L. Kristensen

Harvard-Smithsonian Center for Astrophysics

E. F. van Dishoeck

Universiteit Leiden

Max Planck-institutet

S. Bruderer

Max Planck-institutet

I. San Jose-Garcia

Universiteit Leiden

A. Karska

Max Planck-institutet

R. Visser

University of Michigan

G. Santangelo

Osservatorio Astrofisico di Arcetri

Osservatorio Astronomico di Roma

A. O. Benz

Eidgenössische Technische Hochschule Zürich (ETH)

E. A. Bergin

University of Michigan

P. Caselli

University of Leeds

Max Planck-institutet

F. Herpin

Universite de Bordeaux

Laboratoire d'Astrophysique de Bordeaux

M. R. Hogerheijde

Universiteit Leiden

D. Johnstone

National Research Council Canada

University of Victoria

Joint Astronomy Centre

T. A. van Kempen

Universiteit Leiden

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

B. Nisini

Osservatorio Astronomico di Roma

M. Tafalla

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. 572 24267


Astronomi, astrofysik och kosmologi