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.

line: formation

rich agb stars

molecular inventory

stars: AGB and post-AGB


open clusters

circumstellar water-vapor

stars: individual: W

giant branch stars


semiregular variables


radiative transfer

stars: mass-loss


circumstellar matter

isotope ratios

evolved stars


T. Khouri

Anton Pannekoek Institute for Astronomy

A. de Koter

KU Leuven

Anton Pannekoek Institute for Astronomy

L. Decin

KU Leuven

Anton Pannekoek Institute for Astronomy

Lbfm Waters

Anton Pannekoek Institute for Astronomy

Netherlands Institute for Space Research (SRON)

R. Lombaert

KU Leuven

P. Royer

KU Leuven

B. Swinyard

Rutherford Appleton Laboratory

University College London (UCL)

M. J. Barlow

University College London (UCL)

J. Alcolea

Observatorio Astronómico Nacional (OAN)

Jadl Blommaert

Vrije Universiteit Brüssel (VUB)

KU Leuven

V. Bujarrabal

Observatorio Astronómico Nacional (OAN)

J. Cernicharo

Centro de Astrobiologia (CAB)

M. A. T. Groenewegen

Royal Observatory of Belgium

Kay Justtanont

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

F. Kerschbaum

Universität Wien

Matthias Maercker

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

A. P. Marston

European Space Astronomy Centre (ESAC)

M. Matsuura

University College London (UCL)

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

K. M. Menten


Hans Olofsson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Planesas

Observatorio Astronómico Nacional (OAN)

E. T. Polehampton

Rutherford Appleton Laboratory

University of Lethbridge

T. Posch

Universität Wien

M. Schmidt

Polish Academy of Sciences

R. Szczerba

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



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