Water Vapor in Carbon-rich Asymptotic Giant Branch Stars from the Vaporization of Icy Orbiting Bodies

Abstract

We argue that the presence of water vapor in the circumstellar outflow of a carbon-rich asymptotic giant branch (AGB) star is potentially a distinctive signature of extrasolar cometary systems. Detailed models show that at suitable distances from the star, water ice can survive well into the carbon-rich AGB phase; water vapor abundances as large as 10-6 could result from the vaporization of a collection of orbiting icy bodies with a total mass comparable to what might have been originally present in the solar system's Kuiper Belt. In particular, the recently reported detection by the Submillimeter Wave Astronomy Satellite of water vapor in the circumstellar outflow of IRC +10216 can be explained if ~10 Earth masses of ice is present at a distance ~300 AU from that carbon-rich star. Future observations with the Herschel Space Observatory (HSO, formerly known as the Far Infrared Submillimeter Telescope) will facilitate sensitive multitransition observations of water, yielding line ratios that can establish the radial distribution of water vapor in IRC +10216. The greater sensitivity of HSO will also allow searches for water vapor to be carried out in a much larger sample of carbon-rich AGB stars.