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 \lesssim 20 - 30 stellar radii .