Volume 588, April 2016
|Number of page(s)||41|
|Section||Interstellar and circumstellar matter|
|Published online||30 March 2016|
Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars⋆
1 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
2 KU Leuven, Instituut voor Sterrenkunde, Celestijnenlaan 200D 2401, 3001 Leuven, Belgium
3 University of Amsterdam, Astronomical Institute “Anton Pannekoek”, PO Box 94249, 1090 GE Amsterdam, The Netherlands
4 Universidad de Alcalá de Henares, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
5 Johns Hopkins University, Department of Physics and Astronomy, 3400 North Charles Street, Baltimore, MD 21218, USA
6 Group of Molecular Astrophysics, Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de La Cruz N3, 28049 Cantoblanco, Madrid, Spain
7 Vrije Universiteit Brussel, Department of Physics and Astrophysics, Pleinlaan 2, 1050 Brussels, Belgium
8 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
9 University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
Received: 23 July 2015
Accepted: 12 January 2016
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 Ṁg> 5 × 10-7 M⊙ yr-1, 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.
Key words: stars: AGB and post-AGB / stars: abundances / stars: mass-loss / stars: winds, outflows / stars: carbon
© ESO, 2016
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