The wind of W Hydrae as seen by Herschel I. The CO envelope
Journal article, 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.
radiative-transfer
mu-m
molecular inventory
Hydrae
rich agb stars
line: formation
stars: individual: W
isotope ratios
open clusters
circumstellar matter
feature
stars: AGB and post-AGB
giant branch stars
semiregular variables
radiative transfer
circumstellar water-vapor
evolved stars
stars: mass-loss
Author
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
R. Lombaert
KU Leuven
P. Royer
KU Leuven
B. Swinyard
STFC Rutherford Appleton Laboratory
University College London (UCL)
M. J. Barlow
University College London (UCL)
J. Alcolea
Spanish National Observatory (OAN)
Jadl Blommaert
Vrije Universiteit Brussel (VUB)
KU Leuven
V. Bujarrabal
Spanish National Observatory (OAN)
J. Cernicharo
Centro de Astrobiologia (CAB)
M. A. T. Groenewegen
Royal Observatory of Belgium
Kay Justtanont
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
F. Kerschbaum
University of Vienna
Matthias Maercker
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
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
Max Planck Society
Hans Olofsson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
P. Planesas
Spanish National Observatory (OAN)
E. T. Polehampton
University of Lethbridge
STFC Rutherford Appleton Laboratory
T. Posch
University of Vienna
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- A5Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201322578