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Issue A&A
Volume 422, Number 2, August I 2004
Page(s) 651 - 663
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20040482



A&A 422, 651-663 (2004)
DOI: 10.1051/0004-6361:20040482

Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L $\mathsf{^2}$ Pup

F. L. Schöier1, H. Olofsson1, T. Wong2, 3, M. Lindqvist4 and F. Kerschbaum5

1  Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden
    e-mail: fredrik@astro.su.se
2  CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
3  School of Physics, University of New South Wales, Sydney NSW 2052, Australia
4  Onsala Space Observatory, 439 92 Onsala, Sweden
5  Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Austria

(Received 19 March 2004 / Accepted 8 April 2004 )

Abstract
High angular resolution Australia Telescope Compact Array (ATCA) observations of SiO "thermal" millimetre line emission towards the two oxygen-rich, low mass loss rate AGB stars R Dor and L 2 Pup are presented. In both cases the emission is resolved with an overall spherical symmetry. Detailed radiative transfer modelling of the SiO line emission has been performed, and the comparison between observations and models are conducted in the visibility plane, maximizing the sensitivity. The excitation analysis suggests that the abundance of SiO is as high as $4\times 10^{-5}$ in the inner part of the wind, close to the predicted values from stellar atmosphere models. Beyond a radius of ${\approx} 1\times 10^{15}$ cm the SiO abundance is significantly lower, about $3\times 10^{-6}$, until it decreases strongly at a radius of about $3\times 10^{15}$ cm. This is consistent with a scenario where SiO first freezes out onto dust grains, and then eventually becomes photodissociated by the interstellar UV-radiation field. In these low expansion velocity sources the turbulent broadening of the lines plays an important role in the line formation. Micro-turbulent velocity widths in the range 1.1-1.5 km s -1 result in a very good reproduction of the observed line shapes even if the gas expansion velocity is kept constant. This, combined with the fact that the SiO and CO lines are well fitted using the same gas expansion velocity (to within 5-10%), suggest that the envelope acceleration occurs close to the stellar photosphere, within ${\la} 20{-}30$ stellar radii.


Key words: stars: AGB and post-AGB -- stars: carbon -- stars: late-type -- stars: mass-loss

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© ESO 2004

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