The wind of W Hydrae as seen by Herschel II. The molecular envelope of W Hydrae
Journal article, 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: AGB and post-AGB

stars: winds

radiative transfer

stars: late-type


stars: individual: W Hydrae


T. Khouri

Anton Pannekoek Institute for Astronomy

A. de Koter

Anton Pannekoek Institute for Astronomy

KU Leuven

L. Decin

KU Leuven

Anton Pannekoek Institute for Astronomy

Lbfm Waters

Netherlands Institute for Space Research (SRON)

Anton Pannekoek Institute for Astronomy

Matthias Maercker

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

R. Lombaert

KU Leuven

J. Alcolea

Jadl Blommaert

KU Leuven

Vrije Universiteit Brussel (VUB)

V. Bujarrabal

M. A. T. Groenewegen

Kay Justtanont

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

F. Kerschbaum

University of Vienna

M. Matsuura

University College London (UCL)

K. M. Menten

Max Planck Society

Hans Olofsson

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

P. Planesas

P. Royer

KU Leuven

M. Schmidt

Polish Academy of Sciences

R. Szczerba

Polish Academy of Sciences

D. Teyssier

European Space Astronomy Centre (ESAC)

J. A. Yates

University College London (UCL)

Astronomy and Astrophysics

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

Vol. 570 Art. no. A67- A5

Subject Categories

Astronomy, Astrophysics and Cosmology



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