H2O Formation in C-rich AGB Winds

The Herschel detection of warm H2O vapor emission from C-rich winds of AGB stars challenges the current understanding of circumstellar chemistry. Two mechanisms have been invoked to explain warm H2O formation. In the first, penetration of UV interstellar radiation through a clumpy circumstellar medium causes the formation of H2O molecules in the inner envelope. In the second, periodic shocks passing through the medium immediately above the stellar surface lead to H2O formation. We have identified H2O emission trends from distance-independent line-strength ratios in a sample of 18 C-rich AGB sources, by comparing to a theoretical model grid. We detect warm H2O emission close to or inside the acceleration zone of all sample stars. We find an anti-correlation between the H2O/CO line-strength ratios and the mass-loss rate for Mgas>3×10-7 M⊙/yr. This implies that the H2O formation mechanism becomes less efficient with increasing envelope column density. The anti-correlation breaks down for SRb objects, which clump together at an overall lower H2O abundance. Finally, a radial dependence of the H2O abundance within individual sources is unlikely. These findings lend support to shock-induced non-equilibrium chemistry as the primary source of H2O formation in C-rich AGB stars.