PHOTO-EVAPORATION OF CLUMPS IN PLANETARY-NEBULAE

We study the evolution of dense neutral clumps located in the outer pa rts of planetary nebulae. These clumps will be photo-ionized by the io nizing radiation from the central star and change their structure in t he process. The main effect of the ionization process is the setting u p of a photo-evaporation Bow and a shock running through the clump. On ce this shock has moved through the entire clump it starts to accelera te because of the 'rocket effect'. This continues until the entire clu mp has been photo-ionized. We present an analytic model for the shock and accelerating phases and also the results of numerical simulations which include detailed microphysics. We find a good match between the analytic description and the numerical results and use the numerical r esults to produce some of the clump's observational characteristics at different phases of its evolution. We compare the results with the pr operties of the fast moving low ionization knots (ansae or FLIERs) see n in a number of planetary nebulae. We find that the models match many of the kinematic and emission properties of FLIERs.