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
University of Groningen
Netherlands Institute for Space Research (SRON)
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
Max Planck Society
Leiden University
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
California Institute of Technology (Caltech)
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 Society
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
California Institute of Technology (Caltech)
D. Johnstone
National Research Council Canada
University of Victoria
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 Society
J. C. Pearson
California Institute of Technology (Caltech)
R. Plume
University of Calgary
C. Risacher
Netherlands Institute for Space Research (SRON)
J. Santiago-García
Institut de Radioastronomie Millimétrique (IRAM)
P. Saraceno
Istituto di Fisica dello Spazio Interplanetario, Rome
R. Shipman
Netherlands Institute for Space Research (SRON)
P. Siegel
California Institute of Technology (Caltech)
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 Society
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 521 1 L29Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1051/0004-6361/201015097