“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 data, which contain spectra of several ground vibrational state SiO rotational lines, are presented. 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 $6\times10^{-6}$, and a minimum of $2\times10^{-6}$ and a maximum of $5\times10^{-5}$. The high mass-loss rate Miras have a much lower median abundance, $\la$10-6. The derived SiO abundances are in all cases well below the abundance expected from stellar atmosphere equilibrium 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 is high already at low mass-loss rates and which increases with the mass-loss rate. The high mass-loss rate Miras appear to have a bimodal SiO abundance distribution, a low abundance group (on average $4\times10^{-7}$) and a high abundance group (on average $5\times10^{-6}$). The estimated SiO envelope sizes agree well with the estimated SiO photodissociation radii using an unshielded photodissociation rate of $2.5\times10^{-10}$ s-1. The SiO and CO radio line profiles differ in shape. In general, the SiO line profiles are narrower than the CO line profiles, but they have low-intensity wings which cover the full velocity range of the CO line profile. This is interpreted as partly an effect of selfabsorption in the SiO lines, and partly (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.