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.
open clusters
stars: AGB and post-AGB
stars: individual: W
molecular inventory
mu-m
circumstellar matter
giant branch stars
feature
evolved stars
Hydrae
semiregular variables
rich agb stars
stars: mass-loss
line: formation
radiative transfer
isotope ratios
radiative-transfer
circumstellar water-vapor
Author
T. Khouri
Astronomical Institute Anton Pannekoek
A. de Koter
Astronomical Institute Anton Pannekoek
KU Leuven
L. Decin
Astronomical Institute Anton Pannekoek
KU Leuven
Lbfm Waters
Astronomical Institute Anton Pannekoek
SRON Netherlands Institute for Space Research
R. Lombaert
KU Leuven
P. Royer
KU Leuven
B. Swinyard
UCL
Rutherford Appleton Laboratory
M. J. Barlow
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, Earth and Space Sciences, Radio Astronomy and Astrophysics
F. Kerschbaum
Universitat Wien
Matthias Maercker
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
A. P. Marston
European Space Astronomy Centre
M. Matsuura
UCL
G. J. Melnick
Harvard-Smithsonian Center for Astrophysics
K. M. Menten
Max Planck Institut fur Radioastronomie
Hans Olofsson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
P. Planesas
Observatorio Astronomico Nacional
E. T. Polehampton
Rutherford Appleton Laboratory
University of Lethbridge
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
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