Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars

Autor(en)

Robin Lombaert, Leen Decin, Pierre Royer, Alex de Koter, Nick L. J. Cox, Franz Kerschbaum

Abstrakt

Context. The recent detection of warm H

₂O 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

₂O vapor formation. In the first, periodic shocks passing through the medium immediately above the stellar surface lead to H

₂O formation. In the second, penetration of ultraviolet interstellar radiation through a clumpy circumstellar medium leads to the formation of H

₂O molecules in the intermediate wind. Aims. We aim to determine the properties of H

₂O emission for a sample of 18 carbon-rich AGB stars and subsequently constrain which of the above mechanisms provides the most likely warm H

₂O formation pathway. Methods. Using far-infrared spectra taken with the PACS instrument onboard the Herschel telescope, we combined two methods to identify H

₂O emission trends and interpreted these in terms of theoretically expected patterns in the H

₂O abundance. Through the use of line-strength ratios, we analyzed the correlation between the strength of H

₂O emission and the mass-loss rate of the objects, as well as the radial dependence of the H

₂O 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 H

₂O emission close to or inside the wind acceleration zone of all sample stars, irrespective of their stellar or circumstellar properties. The predicted H

₂O 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 H

₂O/CO line-strength ratio and gas mass-loss rate for ?

_g> 5 × 10

^-7 M

_? yr

^-1, regardless of the upper-level energy of the relevant transitions. This implies that the H

₂O 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 H

₂O formation in carbon-rich AGB stars, although photodissociation by interstellar UV photons may still contribute. Both mechanisms fail in predicting the high H

₂O abundances we infer in Miras and semiregular-a sources, while our results for the semiregular-b objects are inconclusive.

Organisation(en)

Institut für Astrophysik

Externe Organisation(en)

Katholieke Universiteit Leuven, University of Amsterdam (UvA)

Journal

Astronomy & Astrophysics

Band

588

Anzahl der Seiten

41

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201527049

Publikationsdatum

04-2016

Peer-reviewed

Ja

ÖFOS 2012

103003 Astronomie, 103004 Astrophysik

Schlagwörter

ASJC Scopus Sachgebiete

Astronomy and Astrophysics, Space and Planetary Science

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/constraints-on-the-h2o-formation-mechanism-in-the-wind-of-carbonrich-agb-stars(cad2e146-33d9-47b2-bfe5-772ea10bcd47).html