Double-shell circumstellar dust shells

Models are presented for high optical depth Type II OH-IR sources with deep silicate absorption. The models consist of a cool star at the centre of a spherically symmetric |$n(r)\,\propto\,{r}^{-2}$| dust shell together with a second shell of cold absorbing dust. From kinematic arguments the luminosities of the sources are close to |${10}^{5}\,{M}_{\odot}$.| The orgin of the second shell is discussed and the only plausible explanation is that it represents a previous phase of ejection by the star. The mass of gas in the shell of cold dust is estimated to be 4–15M⊙ assuming normal abundances, and it is inferred that the sources must represent the late stages of evolution of massive stars, |$M\,\gtrsim\,30\,{M}_{\odot}$.| The alternative hypothesis, that the stars are giants in the last stages of planetary nebula formation, could hold only if the second shell is at least a factor of 3 closer in to the star than estimated here. This would lead to a poor fit of the long wavelength data.