Letter to the Editor

Discovery of water vapour in the carbon star V Cygni from observations with Herschel/HIFI^*

¹ The Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21218, USA e-mail: neufeld@pha.jhu.edu
² Departamento de Física, Universidad de Alcalá de Henares, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁴ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
⁵ Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain
⁶ Observatorio Astronómico Nacional (IGN), Alfonso XII No. 3, 28014 Madrid, Spain
⁷ CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁹ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
¹⁰ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹¹ Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
¹² Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
¹³ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
¹⁶ Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
¹⁷ University of Waterloo, and University of Western Ontario, Canada
¹⁸ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁹ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile
²⁰ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 28 May 2010
Accepted: 6 July 2010

Abstract

We report the discovery of water vapour toward the carbon star V Cygni. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1₁₁–0₀₀ para-water transition at 1113.3430 GHz in the upper sideband of the Band 4b receiver. The observed spectral line profile is nearly parabolic, but with a slight asymmetry associated with blueshifted absorption, and the integrated antenna temperature is 1.69 ± 0.17 K km s^-1. This detection of thermal water vapour emission, carried out as part of a small survey of water in carbon-rich stars, is only the second such detection toward a carbon-rich AGB star, the first having been obtained by the Submillimeter Wave Astronomy Satellite toward IRC+10216. For an assumed ortho-to-para ratio of 3 for water, the observed line intensity implies a water outflow rate ~3–6 × 10^-5 Earth masses per year and a water abundance relative to H₂ of ~2–5 × 10^-6. This value is a factor of at least 10⁴ larger than the expected photospheric abundance in a carbon-rich environment, and – as in IRC+10216 – raises the intriguing possibility that the observed water is produced by the vapourisation of orbiting comets or dwarf planets. However, observations of the single line observed to date do not permit us to place strong constraints upon the spatial distribution or origin of the observed water, but future observations of additional transitions will allow us to determine the inner radius of the H₂O-emitting zone, and the H₂O ortho-to-para ratio, and thereby to place important constraints upon the origin of the observed water emission.