The widespread occurence of water vapor in the circumstellar envelopes of carbon-rich AGB stars: first results from a survey with Herschel/HIFI

TL;DR

This study reveals that water vapor is commonly present in the circumstellar envelopes of carbon-rich AGB stars, based on Herschel/HIFI observations of eight such stars, challenging previous assumptions about their compositions.

Contribution

First systematic survey demonstrating widespread water vapor in carbon-rich AGB stars using Herschel/HIFI, expanding understanding of their chemical complexity.

Findings

Water vapor detected in all eight surveyed stars.

Water line widths similar to CO lines, suggesting common origin.

No correlation between water outflow rate and total mass-loss rate.

Abstract

We report the preliminary results of a survey for water vapor in a sample of eight C stars with large mid-IR continuum fluxes: V384 Per, CIT 6, V Hya, Y CVn, IRAS 15194-5115, V Cyg, S Cep, and IRC+40540. This survey, performed using the HIFI instrument on board the Herschel Space Observatory, entailed observations of the lowest transitions of both ortho- and para-water: the 556.936 GHz 1(10)-1(01) and 1113.343 GHz 1(11)-0(00) transitions, respectively. Water vapor was unequivocally detected in all eight of the target stars. Prior to this survey, IRC+10216 was the only carbon-rich AGB star from which thermal water emissions had been discovered, in that case with the use of the Submillimeter Wave Astronomy Satellite (SWAS). Our results indicate that IRC+10216 is not unusual, except insofar as its proximity to Earth leads to a large line flux that was detectable with SWAS. The water spectral line widths are typically similar to those of CO rotational lines, arguing against the vaporization of a Kuiper belt analog (Ford & Neufeld 2001) being the general explanation for water vapor in carbon-rich AGB stars. There is no apparent correlation between the ratio of the integrated water line fluxes to the 6.3 micron continuum flux - a ratio which measures the water outflow rate - and the total mass-loss rate for the stars in our sample.