Water vapor toward starless cores: The Herschel view
Artikel i vetenskaplig tidskrift, 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.

Författare

P. Caselli

Osservatorio Astrofisico di Arcetri

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, Kapteyn Astronomical Institute

SRON Netherlands Institute for Space Research

M. Tafalla

Observatorio Astronomico Nacional

E. A. Bergin

University of Michigan

B. Nisini

Osservatorio Astronomico di Roma

C. Codella

Osservatorio Astrofisico di Arcetri

E. F. van Dishoeck

Max Planck-institutet

Leiden University

R. Bachiller

Observatorio Astronomico Nacional

A. Baudry

Laboratoire d'Astrophysique de Bordeaux

M. Benedettini

Istituto di Fisica dello Spazio Interplanetario, Rome

A. O. Benz

Eidgenössische Technische Hochschule Zürich (ETH)

Per Bjerkeli

Chalmers, Institutionen för radio- och rymdvetenskap, Radioastronomi och astrofysik

G. A. Blake

Caltech Division of Geological and Planetary Sciences

S. Bontemps

Laboratoire d'Astrophysique de Bordeaux

J. Braine

Laboratoire d'Astrophysique de Bordeaux

S. Bruderer

Eidgenössische Technische Hochschule Zürich (ETH)

J. Cernicharo

CSIC-INTA - Centro de Astrobiologia (CAB)

F. Daniel

CSIC-INTA - Centro de Astrobiologia (CAB)

A. M. di Giorgio

Istituto di Fisica dello Spazio Interplanetario, Rome

C. Dominik

Astronomical Institute Anton Pannekoek

Radboud Universiteit

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

Observatorio Astronomico Nacional

T. Gaier

Jet Propulsion Laboratory, California Institute of Technology

T. Giannini

Osservatorio Astronomico di Roma

J. R. Goicoechea

CSIC-INTA - Centro de Astrobiologia (CAB)

T. de Graauw

SRON Netherlands Institute for Space Research

F. Helmich

SRON Netherlands Institute for Space Research

G. J. Herczeg

Max Planck-institutet

F. Herpin

Laboratoire d'Astrophysique de Bordeaux

M. R. Hogerheijde

Leiden University

B. Jackson

SRON Netherlands Institute for Space Research

T. Jacq

Osservatorio Astrofisico di Arcetri

H. Javadi

Jet Propulsion Laboratory, California Institute of Technology

D. Johnstone

University of Victoria

National Research Council Canada

J. K. Jorgensen

Kobenhavns Universitet

D. Kester

SRON Netherlands Institute for Space Research

L. Kristensen

Leiden University

W. Laauwen

SRON Netherlands Institute for Space Research

B. Larsson

Stockholms universitet

D. Lis

CalTech

René Liseau

Chalmers, Institutionen för radio- och rymdvetenskap, Radioastronomi och astrofysik

W. Luinge

SRON Netherlands Institute for Space Research

M. Marseille

SRON Netherlands Institute for Space Research

C. McCoey

Western University

University of Waterloo

A. Megej

Eidgenössische Technische Hochschule Zürich (ETH)

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

D. A. Neufeld

Johns Hopkins University

Michael Olberg

Chalmers, Institutionen för radio- och rymdvetenskap, Nationella anläggningen för radioastronomi

B. Parise

Max Planck-institutet

J. C. Pearson

Jet Propulsion Laboratory, California Institute of Technology

R. Plume

University of Calgary

C. Risacher

SRON Netherlands Institute for Space Research

J. Santiago-García

Instituto de Radioastronomia Milimetrica

P. Saraceno

Istituto di Fisica dello Spazio Interplanetario, Rome

R. Shipman

SRON Netherlands Institute for Space Research

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

Eidgenössische Technische Hochschule Zürich (ETH)

F. Wyrowski

Max Planck-institutet

Astronomy and Astrophysics

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

Vol. 521 L29

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

DOI

10.1051/0004-6361/201015097