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

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 L2 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 4E-5 in the inner part of the wind, close to the predicted values from stellar atmosphere models. Beyond a radius of ~ 1E15 cm the SiO abundance is significantly lower, about 3E-6, until it decreases strongly at a radius of about 3E15 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 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 20-30 stellar radii.