The influence of dust upon the dynamics and thermal stability of planetary nebulae

It is argued that no significant amount of dust could have existed in a planetary nebula until the radius of the shell exceeded 1014 cm. Otherwise, radiation force acting on the grains would accelerate the shell to speeds well in excess of the observed expansion rates. However, if grain formation occurs before the radius of the shell is 1017 cm, the bulk of the nebula will be shielded from the ultraviolet radiation from the central star. Then, conditions would be favorable for the development of thermal instabilities, which could result in the formation of the inhomogeneities observed in planetary nebulae. Subject headings: planetary nebulae - radiative transfer