‘Thermal’ SiO radio line emission towards M-type AGB stars: a probe of circumstellar dust formation and dynamics⋆

An extensive radiative transfer analysis of circumstellar SiO ‘thermal’ radio line emission from a large sample of M-type AGB stars has been performed. The sample contains 18 irregulars of type Lb (IRV), 7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12 Miras. New observational material, which contains data in several SiO rotational lines in the ground vibrational state, is presented for the IRV/SRVs. The detection rate was about 60% (44% for the IRVs, and 68% for the SRVs). SiO fractional abundances have been determined through radiative transfer modelling. The abundance distribution of the IRV/SRV sample has a median value of 8×10, and a minimum of 1×10 and a maximum of 5×10. The high mass loss rate Miras have a bimodal abundance distribution, a low abundance group (on average 4×10) and a high abundance group (on average 7×10). The derived SiO abundances are in all cases well below the abundance expected from stellar atmosphere chemistry, on average by a factor of ten. In addition, there is a trend of decreasing SiO abundance with increasing mass loss rate. This is interpreted in terms of depletion of SiO molecules by the formation of silicate grains in the circumstellar envelopes, with an efficiency which increases with the mass loss rate. At this point we cannot give a definite explanation to why the high mass loss rate Miras separate into two distinct abundance groups. The SiO and CO radio line profiles differ in shape. In general, the dominating part of the SiO profiles are narrower than the CO profiles, but they have low-intensity wings which cover the full velocity range of the CO profile. This is interpreted (as has been done also by others) as due to the influence of gas acceleration in the region which produces a significant fraction of the SiO line emission. Finally, a number of sources which have peculiar CO line profiles are discussed from the point of view of their SiO line properties.