Constraints on the H2O formation mechanism in the wind of carbon-rich
  AGB stars
  Context. The recent detection of warm H$_2$O vapor emission from the outflows
of carbon-rich asymptotic giant branch (AGB) stars challenges the current
understanding of circumstellar chemistry. Two mechanisms have been invoked to
explain warm H$_2$O vapor formation. In the first, periodic shocks passing
through the medium immediately above the stellar surface lead to H$_2$O
formation. In the second, penetration of ultraviolet interstellar radiation
through a clumpy circumstellar medium leads to the formation of H$_2$O
molecules in the intermediate wind.
  Aims. We aim to determine the properties of H$_2$O emission for a sample of
18 carbon-rich AGB stars and subsequently constrain which of the above
mechanisms provides the most likely warm H$_2$O formation pathway.
  Methods, Results, and Conclusions. See paper.
