Volume 570, October 2014
|Number of page(s)||18|
|Section||Interstellar and circumstellar matter|
|Published online||20 October 2014|
The wind of W Hydrae as seen by Herschel
II. The molecular envelope of W Hydrae⋆
1 Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
2 Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D B-2401, 3001 Leuven, Belgium
3 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
5 Argelander Institute für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
6 Observatorio Astronómico Nacional (IGN), Alfonso XII N°3, 28014 Madrid, Spain
7 Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
8 Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain
9 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
10 University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
11 Dept. of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
12 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
13 N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
14 European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
Received: 28 May 2014
Accepted: 21 August 2014
Context. The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies.
Aims. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the 12CO molecule. Here we broaden this study by including an extensive set of H2O and 28SiO lines. It is the first time such a comprehensive study is performed for this source. The oxygen isotopic ratios and the 28SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively.
Methods. We model the molecular emission observed by the three instruments on board Herschel Space Observatory using a state-of-the-art molecular excitation and radiative transfer code. We also account for the dust radiation field in our calculations.
Results. We find an H2O ortho-to-para ratio of 2.5 +2.5-1.0, consistent with what is expected for an AGB wind. The O16/O17 ratio indicates that W Hya has an initial mass of about 1.5 M⊙. Although the ortho- and para-H2O lines observed by HIFI appear to trace gas of slightly different physical properties, we find that a turbulence velocity of 0.7 ± 0.1 km s-1 fits the HIFI lines of both spin isomers and those of 28SiO well.
Conclusions. The modelling of H2O and 28SiO confirms the properties of the envelope model of W Hya, as derived from 12CO lines, and allows us to constrain the turbulence velocity. The ortho- and para-H216O and 28SiO abundances relative to H2 are (6+ 3-2)×10-4, (3+ 2-1)×10-4, and (3.3 ± 0.8) × 10-5, respectively, in agreement with expectations for oxygen-rich AGB outflows. Assuming a solar silicon-to-carbon ratio, the 28SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.
Key words: stars: AGB and post-AGB / radiative transfer / stars: late-type / stars: individual: W Hydrae / stars: winds, outflows
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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