Letter to the Editor

Water vapor toward starless cores: The Herschel view^*

¹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK e-mail: p.caselli@leeds.ac.uk
² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
⁴ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁵ Department of Earth and Planetary Sciences, Kobe University, Nada, Kobe 657-8501, Japan
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁹ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain
¹⁰ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹¹ INAF - Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
¹² Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
¹³ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
¹⁴ INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁵ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹⁶ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁷ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹⁸ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
²⁰ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²¹ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
²² University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
²³ Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain
²⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁵ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²⁶ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
²⁷ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
²⁸ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²⁹ California Institute of Technology, Cahill Center for Astronomy and Astrophysics, MS 301-17, Pasadena, CA 91125, USA
³⁰ the University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada
³¹ Microwave Laboratory, ETH Zurich, 8092 Zurich, Switzerland
³² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁴ 
Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
³⁵ Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain

Received: 31 May 2010
Accepted: 18 June 2010

Abstract

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 km s^-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)  5 × 10^-9 at radii >7000 AU and 2 × 10^-10 toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H₂O) profile peaking at 10^-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.