The wind of W Hydrae as seen by Herschel I. The CO envelope
Artikel i vetenskaplig tidskrift, 2014

Context. Asymptotic giant branch (AGB) stars lose their envelopes by means of a stellar wind whose driving mechanism is not understood well. Characterizing the composition and thermal and dynamical structure of the outflow provides constraints that are essential for understanding AGB evolution, including the rate of mass loss and isotopic ratios. Aims. We characterize the CO emission from the wind of the low mass-loss rate oxygen-rich AGB star W Hya using data obtained by the HIFI, PACS, and SPIRE instruments on board the Herschel Space Observatory and ground-based telescopes. (CO)-C-12 and (CO)-C-13 lines are used to constrain the intrinsic C-12/C-13 ratio from resolved HIFI lines. Methods. We combined a state-of-the-art molecular line emission code and a dust continuum radiative transfer code to model the CO lines and the thermal dust continuum. Results. The acceleration of the outflow up to about 5.5 km s(-1) is quite slow and can be represented by a beta-type velocity law with index beta = 5. Beyond this point, acceleration up the terminal velocity of 7 km s(-1) is faster. Using the J = 10-9, 9-8, and 6-5 transitions, we find an intrinsic C-12/C-13 ratio of 18 +/- 10 for W Hya, where the error bar is mostly due to uncertainties in the (CO)-C-12 abundance and the stellar flux around 4.6 mu m. To match the low-excitation CO lines, these molecules need to be photo-dissociated at similar to 500 stellar radii. The radial dust emission intensity profile of our stellar wind model matches PACS images at 70 mu m out to 20 '' (or 800 stellar radii). For larger radii the observed emission is substantially stronger than our model predicts, indicating that at these locations there is extra material present. Conclusions. The initial slow acceleration of the wind may imply inefficient dust formation or dust driving in the lower part of the envelope. The final injection of momentum in the wind might be the result of an increase in the opacity thanks to the late condensation of dust species. The derived intrinsic isotopologue ratio for W Hya is consistent with values set by the first dredge-up and suggestive of an initial mass of 2 M-circle dot or more. However, the uncertainty in the isotopologic ratio is large, which makes it difficult to set reliable limits on W Hya's main-sequence mass.

open clusters

circumstellar matter

molecular inventory


giant branch stars


line: formation

radiative transfer

circumstellar water-vapor

stars: individual: W

stars: AGB and post-AGB

evolved stars

isotope ratios


rich agb stars

semiregular variables

stars: mass-loss



T. Khouri

Astronomical Institute Anton Pannekoek

A. de Koter

Astronomical Institute Anton Pannekoek

KU Leuven

L. Decin

KU Leuven

Astronomical Institute Anton Pannekoek

Lbfm Waters

SRON Netherlands Institute for Space Research

Astronomical Institute Anton Pannekoek

R. Lombaert

KU Leuven

P. Royer

KU Leuven

B. Swinyard

University College London (UCL)

Rutherford Appleton Laboratory

M. J. Barlow

University College London (UCL)

J. Alcolea

Observatorio Astronomico Nacional

Jadl Blommaert

Vrije Universiteit Brussel

KU Leuven

V. Bujarrabal

Observatorio Astronomico Nacional

J. Cernicharo

CSIC-INTA - Centro de Astrobiologia (CAB)

M. A. T. Groenewegen

Royal Observatory of Belgium

Kay Justtanont

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

F. Kerschbaum

Universitat Wien

Matthias Maercker

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

A. P. Marston

European Space Astronomy Centre

M. Matsuura

University College London (UCL)

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

K. M. Menten

Max Planck-institutet

Hans Olofsson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Planesas

Observatorio Astronomico Nacional

E. T. Polehampton

University of Lethbridge

Rutherford Appleton Laboratory

T. Posch

Universitat Wien

M. Schmidt

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

R. Szczerba

Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

B. Vandenbussche

KU Leuven

J. A. Yates

University College London (UCL)

Astronomy and Astrophysics

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

Vol. 561 Article no. A5- A5


Astronomi, astrofysik och kosmologi