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High angular resolution Australia Telescope Compact Array (ATCA) observationsof SiO `thermal' millimetre line emission towards the two oxygen-rich, low massloss rate AGB stars R Dor and L2 Pup are presented. In both cases the emissionis resolved with an overall spherical symmetry. Detailed radiative transfermodelling of the SiO line emission has been performed, and the comparisonbetween observations and models are conducted in the visibility plane,maximizing the sensitivity. The excitation analysis suggests that the abundanceof SiO is as high as 4E-5 in the inner part of the wind, close to the predictedvalues from stellar atmosphere models. Beyond a radius of ~ 1E15 cm the SiOabundance is significantly lower, about 3E-6, until it decreases strongly at aradius of about 3E15 cm. This is consistent with a scenario where SiO firstfreezes out onto dust grains, and then eventually becomes photodissociated bythe interstellar UV-radiation field. In these low expansion velocity sourcesthe turbulent broadening of the lines plays an important role in the lineformation. Micro-turbulent velocity widths in the range 1.1-1.5 km/s result ina very good reproduction of the observed line shapes even if the gas expansionvelocity is kept constant. This, combined with the fact that the SiO and COlines 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 20-30 stellar radii.Comment: Accepted for publication in A&A, 14 pages, 14 figure

astronomy and astrophysics

Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L<sup>2</sup> Pup

Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L2 Pup

Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L² Pup