2016A&A...588A.124L

Context. The recent detection of warm H₂O 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 H₂O vapor formation. In the first, periodic shocks passing through the medium immediately above the stellar surface lead to H₂O formation. In the second, penetration of ultraviolet interstellar radiation through a clumpy circumstellar medium leads to the formation of H₂O molecules in the intermediate wind.
Aims. We aim to determine the properties of H₂O emission for a sample of 18 carbon-rich AGB stars and subsequently constrain which of the above mechanisms provides the most likely warm H₂O formation pathway.
Methods. Using far-infrared spectra taken with the PACS instrument onboard the Herschel telescope, we combined two methods to identify H₂O emission trends and interpreted these in terms of theoretically expected patterns in the H₂O abundance. Through the use of line-strength ratios, we analyzed the correlation between the strength of H₂O emission and the mass-loss rate of the objects, as well as the radial dependence of the H₂O abundance in the circumstellar outflow per individual source. We computed a model grid to account for radiative-transfer effects in the line strengths.
Results. We detect warm H₂O emission close to or inside the wind acceleration zone of all sample stars, irrespective of their stellar or circumstellar properties. The predicted H₂O abundances in carbon-rich environments are in the range of 10^–6 up to 10^–4 for Miras and semiregular-a objects, and cluster around 10^–6 for semiregular-b objects. These predictions are up to three orders of magnitude greater than what is predicted by state-of-the-art chemical models. We find a negative correlation between the H₂O/CO line-strength ratio and gas mass-loss rate for Mg>5x10^–7M☉/yr, regardless of the upper-level energy of the relevant transitions. This implies that the H₂O formation mechanism becomes less efficient with increasing wind density. The negative correlation breaks down for the sources of lowest mass-loss rate, the semiregular-b objects.
Conclusions. Observational constraints suggest that pulsationally induced shocks play an important role in warm H₂O formation in carbon-rich AGB stars, although photodissociation by interstellar UV photons may still contribute. Both mechanisms fail in predicting the high H₂O abundances we infer in Miras and semiregular-a sources, while our results for the semiregular-b objects are inconclusive.