||H2O Formation in C-rich AGB Winds
||497, Why Galaxies Care about AGB Stars III: A Closer Look in Space and Time
||Lombaert, R.; Decin, L.; Royer, P.; de Koter, A.; Cox, N. L. J.; De Ridder, J.; Khouri, T.; Agúndez, M.; Blommaert, J. A. D. L.; Cernicharo, J.; González-Alfonso, E.; Groenewegen, M. A. T.; Kerschbaum, F.; Neufeld, D.; Vandenbussche, B.; Waelkens, C.
||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
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.