The wind of W Hydrae as seen by Herschel. I. The CO envelope

Autor(en)

T. Khouri, A. de Koter, L. Decin, L. B. F. M. Waters, R. Lombaert, P. Royer, B. Swinyard, M. J. Barlow, J. Alcolea, J. A. D. L. Blommaert, V. Bujarrabal, J. Cernicharo, M. A. T. Groenewegen, K. Justtanont, Franz Kerschbaum, M. Maercker, A. Marston, M. Matsuura, G. Melnick, K. M. Menten, H. Olofsson, P. Planesas, E. Polehampton, Thomas Posch, R. Szczerba, B. Vandenbussche, J. Yates

Abstrakt

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.

¹²CO and

¹³CO lines are used to constrain the intrinsic

¹²C/

¹³C 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 β-type velocity law with index β = 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

¹²C/

¹³C ratio of 18 ± 10 for W Hya, where the error bar is mostly due to uncertainties in the

¹²CO abundance and the stellar flux around 4.6 μm. To match the low-excitation CO lines, these molecules need to be photo-dissociated at ~500 stellar radii. The radial dust emission intensity profile of our stellar wind model matches PACS images at 70 μm 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

_⊙ 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.

Organisation(en)

Institut für Astrophysik

Externe Organisation(en)

INAF - Osservatorio Astrofisico di Arcetri, University of Amsterdam (UvA), Katholieke Universiteit Leuven, SRON Netherlands Institute for Space Research , Rutherford Appleton Laboratory, University College London, Observatorio Astronómico Nacional (OAN-IGN), Vrije Universiteit Brussel, Spanish National Research Council (CSIC), Royal Observatory of Belgium, Chalmers University of Technology, European Southern Observatory (Germany), European Space Astronomy Centre (ESA), Max-Planck-Institut für Radioastronomie, Stockholm University, University of Lethbridge, Polish Academy of Sciences (PAS)

Journal

Astronomy & Astrophysics

Band

561

Anzahl der Seiten

13

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201322578

Publikationsdatum

01-2014

Peer-reviewed

Ja

ÖFOS 2012

103003 Astronomie, 103004 Astrophysik

Schlagwörter

ASJC Scopus Sachgebiete

Astronomy and Astrophysics, Space and Planetary Science

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/8a44773f-5096-4e1f-9fd4-415b7b390dd3