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

摘要

We report the discovery of water vapour toward the carbon starVCygni. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 111-000para-watertransition at 1113.3430GHz in the upper sideband of theBand4breceiver. The observed spectral line profile isnearly parabolic, but with a slight asymmetry associated withblueshifted absorption, and the integrated antenna temperature is1.690.17Kkms-1.This detection of thermal water vapour emission, carried out as part ofa small survey of water in carbon-rich stars, isonly the secondsuch detection toward a carbon-rich AGBstar, thefirsthaving been obtained by the Submillimeter Wave Astronomy Satellitetoward IRC+10216. For an assumed ortho-to-para ratio of3 forwater, the observed line intensity implies a water outflow rate 10-5Earth masses per year and a water abundance relative toH2 of 10-6. This value is a factor of at least 104largerthan the expected photospheric abundance in a carbon-rich environment,and- as in IRC+10216- raises the intriguing possibilitythat the observed water is produced by the vapourisation of orbitingcomets or dwarf planets. However, observations of the single lineobserved to date do not permit us to place strong constraints upon thespatial distribution or origin of the observed water, but futureobservations of additional transitions will allow us to determine theinner 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. Key words: stars: AGB and post-AGB - circumstellar matter - submillimeter: stars