Dust properties of planetary nebulae

Shells of planetary nebulae (PNe) are usually modeled applying photoionization only although there is a significant fraction of dust embedded in the nebular shells. The calculated temperature and dust-to-gas ratio from IRAS colors is "faked" by the IR flux continuum contribution from the gas and by transiently heated dust grains. Also the effects of grain growth time and thus maximum grain size lead to strong deviations from the model input values, adopting the IRAS data fits.