The wind of W Hydrae as seen by Herschel

I. The CO envelope^⋆

¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE, Amsterdam, The Netherlands
e-mail: theokhouri@gmail.com
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D 2401, 3001 Leuven, Belgium
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
⁵ Dept. of Physics & Astronomy, University College London, Gower St., London WC1E 6BT, UK
⁶ Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
⁷ Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁸ Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain
⁹ Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussels, Belgium
¹¹ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹² University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
¹³ European Southern Observatory, Karl Schwarzschild Str. 2, 85748 Garching bei München, Germany
¹⁴ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138, USA
¹⁶ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
¹⁷ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
¹⁸ Institute for Space Imaging Science, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1J 1B1, Canada
¹⁹ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland

Received: 30 August 2013
Accepted: 3 October 2013

Abstract

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.