Properties and evolution of dust grains in planetary nebulae

Abstract

The IRAS data of 233 planetary nebulae (PNs) determining the properties of the dust contained in their envelopes have been analyzed. The main result is that the dust grains undergo substantial evolution during the lifetime of a PN. In particular, the grain size and the total dust mass decrease systematically with the nebular radius whereas the number of emitting grains increases. Two scenarios which can explain these results are discussed. A first possibility is that grains undergo processes which both 'erode' and 'shatter' them. It is argued that sputtering by fast atoms of a stellar wind and grain-grain collisions may plausibly produce such effects. Alernatively, the dust present in PNs may have a size distribution since the very beginning and large grains may be selectively destroyed during the PN evolution.