Water vapor toward starless cores: The Herschel view
Journal article, 2010

Aims: Previous studies by the satellites SWAS and Odin provided stringent upper limits on the gas phase water abundance of dark clouds (x(H2O) < 7 × 10-9). We investigate the chemistry of water vapor in starless cores beyond the previous upper limits using the highly improved angular resolution and sensitivity of Herschel and measure the abundance of water vapor during evolutionary stages just preceding star formation. Methods: High spectral resolution observations of the fundamental ortho water (o-H2O) transition (557 GHz) were carried out with the Heterodyne Instrument for the Far Infrared onboard Herschel toward two starless cores: Barnard 68 (hereafter B68), a Bok globule, and LDN 1544 (L1544), a prestellar core embedded in the Taurus molecular cloud complex. Detailed radiative transfer and chemical codes were used to analyze the data. Results: The RMS in the brightness temperature measured for the B68 and L1544 spectra is 2.0 and 2.2 mK, respectively, in a velocity bin of 0.59 kms-1. The continuum level is 3.5 ± 0.2 mK in B68 and 11.4 ± 0.4 mK in L1544. No significant feature is detected in B68 and the 3σ upper limit is consistent with a column density of o-H2O N(o-H2O) < 2.5 × 1013 cm-2, or a fractional abundance x(o-H2O) < 1.3 × 10-9, more than an order of magnitude lower than the SWAS upper limit on this source. The L1544 spectrum shows an absorption feature at a 5σ level from which we obtain the first value of the o-H2O column density ever measured in dark clouds: N(o-H2O) = (8 ± 4) * 1012 cm-2. The corresponding fractional abundance is x(o-H2O) \sime 5 × 10-9 at radii >7000 AU and \sime2 × 10-10 toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H2O) profile peaking at \sime10-8, 0.1 pc away from the core center, where both freeze-out and photodissociation are negligible. Conclusions: Herschel has provided the first measurement of water vapor in dark regions. Column densities of o-H2O are low, but prestellar cores such as L1544 (with their high central densities, strong continuum, and large envelopes) appear to be very promising tools to finally shed light on the solid/vapor balance of water in molecular clouds and oxygen chemistry in the earliest stages of star formation. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Author

P. Caselli

Arcetri Astrophysical Observatory

University of Leeds

E. Keto

Harvard-Smithsonian Center for Astrophysics

L. Pagani

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

Y. Aikawa

Kobe University

U. A. Yildiz

Leiden University

F. F. S. van der Tak

Netherlands Institute for Space Research (SRON)

University of Groningen

M. Tafalla

Spanish National Observatory (OAN)

E. A. Bergin

University of Michigan

B. Nisini

Osservatorio Astronomico di Roma

C. Codella

Arcetri Astrophysical Observatory

E. F. van Dishoeck

Leiden University

Max Planck Institute

R. Bachiller

Spanish National Observatory (OAN)

A. Baudry

Laboratoire d'Astrophysique de Bordeaux

M. Benedettini

Istituto di Fisica dello Spazio Interplanetario, Rome

A. O. Benz

Swiss Federal Institute of Technology in Zürich (ETH)

Per Bjerkeli

Chalmers, Department of Radio and Space Science, Radio Astronomy and Astrophysics

G. A. Blake

California Institute of Technology (Caltech)

S. Bontemps

Laboratoire d'Astrophysique de Bordeaux

J. Braine

Laboratoire d'Astrophysique de Bordeaux

S. Bruderer

Swiss Federal Institute of Technology in Zürich (ETH)

J. Cernicharo

Centro de Astrobiologia (CAB)

F. Daniel

Centro de Astrobiologia (CAB)

A. M. di Giorgio

Istituto di Fisica dello Spazio Interplanetario, Rome

C. Dominik

Radboud University

Anton Pannekoek Institute for Astronomy

S. Doty

Denison University

P. Encrenaz

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

M. Fich

University of Waterloo

A. Fuente

Spanish National Observatory (OAN)

T. Gaier

Jet Propulsion Laboratory, California Institute of Technology

T. Giannini

Osservatorio Astronomico di Roma

J. R. Goicoechea

Centro de Astrobiologia (CAB)

T. de Graauw

Netherlands Institute for Space Research (SRON)

F. Helmich

Netherlands Institute for Space Research (SRON)

G. J. Herczeg

Max Planck Institute

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

M. R. Hogerheijde

Leiden University

B. Jackson

Netherlands Institute for Space Research (SRON)

T. Jacq

Arcetri Astrophysical Observatory

H. Javadi

Jet Propulsion Laboratory, California Institute of Technology

D. Johnstone

University of Victoria

National Research Council Canada

J. K. Jorgensen

University of Copenhagen

D. Kester

Netherlands Institute for Space Research (SRON)

L. Kristensen

Leiden University

W. Laauwen

Netherlands Institute for Space Research (SRON)

B. Larsson

Stockholm University

D. Lis

California Institute of Technology (Caltech)

René Liseau

Chalmers, Department of Radio and Space Science, Radio Astronomy and Astrophysics

W. Luinge

Netherlands Institute for Space Research (SRON)

M. Marseille

Netherlands Institute for Space Research (SRON)

C. McCoey

University of Waterloo

Western University

A. Megej

Swiss Federal Institute of Technology in Zürich (ETH)

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

D. A. Neufeld

Johns Hopkins University

Michael Olberg

Chalmers, Department of Radio and Space Science, National Facility for Radio Astronomy

B. Parise

Max Planck Institute

J. C. Pearson

Jet Propulsion Laboratory, California Institute of Technology

R. Plume

University of Calgary

C. Risacher

Netherlands Institute for Space Research (SRON)

J. Santiago-García

Instituto de Radioastronomía Milimétrica

P. Saraceno

Istituto di Fisica dello Spazio Interplanetario, Rome

R. Shipman

Netherlands Institute for Space Research (SRON)

P. Siegel

Jet Propulsion Laboratory, California Institute of Technology

T. A. van Kempen

Harvard-Smithsonian Center for Astrophysics

R. Visser

Leiden University

S. F. Wampfler

Swiss Federal Institute of Technology in Zürich (ETH)

F. Wyrowski

Max Planck Institute

Astronomy and Astrophysics

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

Vol. 521 1 L29

Subject Categories

Astronomy, Astrophysics and Cosmology

Roots

Basic sciences

DOI

10.1051/0004-6361/201015097

More information

Latest update

5/23/2019