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

TL;DR

This paper reports the first detection of water vapour in the carbon star V Cygni using Herschel/HIFI, suggesting possible origins such as vaporization of orbiting bodies, and highlights the need for further observations to understand its source.

Contribution

It presents the second detection of water vapour in a carbon-rich AGB star and discusses implications for water origin and future observational strategies.

Findings

Detected water vapour emission in V Cygni with Herschel/HIFI.

Water abundance exceeds photospheric expectations by over 10,000 times.

Suggests possible vaporization of orbiting comets or dwarf planets as water source.

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.