Water Vapor in Carbon-rich AGB 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 AGB star is potentially a distinctive signature of extra-solar 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 FIRST) will facilitate sensitive multi-transition 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.