SIMBAD references

We study the evolution of dense neutral clumps located in the outer parts of planetary nebulae. These clumps will be photo-ionized by the ionizing radiation from the central star and change their structure in the process. The main effect of the ionization process is the setting up of a photo-evaporation flow and a shock running through the clump. Once this shock has moved through the entire clump it starts to accelerate because of the `rocket effect'. This continues until the entire clump 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 results to produce some of the clump's observational characteristics at different phases of its evolution. We compare the results with the properties of the fast moving low ionization knots (ansae or FLIERs) seen in a number of planetary nebulae. We find that the models match many of the kinematic and emission properties of FLIERs.