The wind of W Hydrae as seen by Herschel II. The molecular envelope of W Hydrae
Artikel i vetenskaplig tidskrift, 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 (CO)-C-12 molecule. Here we broaden this study by including an extensive set of H2O and (SiO)-Si-28 lines. It is the first time such a comprehensive study is performed for this source. The oxygen isotopic ratios and the (SiO)-Si-28 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(-1.0)(+2.5), 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-circle dot. 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 (SiO)-Si-28 well. Conclusions. The modelling of H2O and (SiO)-Si-28 confirms the properties of the envelope model of W Hya, as derived from (CO)-C-12 lines, and allows us to constrain the turbulence velocity. The ortho-and para-(H2O)-O-16 and (SiO)-Si-28 abundances relative to H-2 are (6(2)(+3)) x 10(-4), (3(-1)(+2)) x 10(-4), and (3.3 +/- 0.8) x 10(-5), respectively, in agreement with expectations for oxygen-rich AGB outflows. Assuming a solar silicon-to-carbon ratio, the (SiO)-Si-28 line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.

stars: late-type


radiative transfer

stars: individual: W Hydrae

stars: winds

stars: AGB and post-AGB


T. Khouri

Astronomical Institute Anton Pannekoek

A. de Koter

KU Leuven

Astronomical Institute Anton Pannekoek

L. Decin

KU Leuven

Astronomical Institute Anton Pannekoek

Lbfm Waters

Astronomical Institute Anton Pannekoek

SRON Netherlands Institute for Space Research

Matthias Maercker

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

R. Lombaert

KU Leuven

J. Alcolea

Jadl Blommaert

Vrije Universiteit Brussel

KU Leuven

V. Bujarrabal

M. A. T. Groenewegen

Kay Justtanont

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

F. Kerschbaum

Universitat Wien

M. Matsuura


K. M. Menten

Max Planck-institutet

Hans Olofsson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Planesas

P. Royer

KU Leuven

M. Schmidt

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

R. Szczerba

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

D. Teyssier

European Space Astronomy Centre

J. A. Yates


Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 570 Art. no. A67- A5


Astronomi, astrofysik och kosmologi