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

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 1(11) - 0(00) 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 \pm 0.17 K km/s. 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) E-5 Earth masses per year and a water abundance relative to H2 of ~ (2-5) E-6. This value is a factor of at least 1E+4 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.

arXiv commentAccepted for publication in A&A (HIFI special issue)

References in corpus (6)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• Warm water vapour in the sooty outflow from a luminous carbon star
• Water Vapor in Carbon-rich AGB Stars from the Vaporization of Icy Orbiting Bodies
• Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry
• Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations

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• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• Photodissociation Region Diagnostics Across Galactic Environments
• Water in IRC+10216: a genuine formation process by shock-induced chemistry in the inner wind
• The widespread occurence of water vapor in the circumstellar envelopes of carbon-rich AGB stars: first results from a survey with Herschel/HIFI
• Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
• Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
• Chemistry in AGB stars: successes and challenges
• A HIFI preview of warm molecular gas around chi Cyg : first detection of H2O emission toward an S-type AGB star
• Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes