The wind of W Hydrae as seen by Herschel. II. The molecular envelope of W Hydrae

Author(s)

T. Khouri, Alex de Koter, Leen Decin, Franz Kerschbaum, L. B. F. M. Waters, Matthias Maercker, Robin Lombaert, J. Alcolea, Joris A D L Blommaert, V. Bujarrabal, Martin A. T. Groenewegen, Kay Justtanont, Mikako Matsuura, K. M. Menten, H. Olofsson, P. Planesas, Pierre Royer, M. R. Schmidt, R. Szczerba, D. Teyssier, Jeremy Yates

Abstract

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 H

₂O 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 H

₂O ortho-to-para ratio of 2.5

^+2.5

_-1.0, consistent with what is expected for an AGB wind. The O

¹⁶/O

¹⁷ ratio indicates that W Hya has an initial mass of about 1.5 M. Although the ortho-and para-H

₂O 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 H

₂O 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-H

₂

¹⁶O and

²⁸SiO abundances relative to H

₂ 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.

Organisation(s)

Department of Astrophysics

External organisation(s)

University of Amsterdam (UvA), Katholieke Universiteit Leuven, SRON Netherlands Institute for Space Research , Polish Academy of Sciences (PAS), Observatorio Astronómico Nacional (OAN-IGN), Chalmers University of Technology, Rheinische Friedrich-Wilhelms-Universität Bonn, Vrije Universiteit Brussel, Royal Observatory of Belgium, University College London, Max-Planck-Institut für Radioastronomie, European Space Astronomy Centre (ESA)

Journal

Astronomy & Astrophysics

Volume

570

No. of pages

18

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201424298

Publication date

10-2014

Peer reviewed

Yes

Austrian Fields of Science 2012

103003 Astronomy, 103004 Astrophysics

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/ab24ef2c-bbd7-451c-8709-5f705fd22ec8

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