  We present a model to account for the observed debris disks around young
white dwarfs and the presence of metal-lines in their spectra. Stellar
evolution models predict that the mass-loss on the AGB will be pulsed;
furthermore, observations indicate that the bulk of the mass-loss occurs on the
AGB. In this case, if the progenitors of the white dwarfs had remnants of
planetary formation like the Sun's Oort cloud or the Kuiper Belt and a planet
lying within that cloud or nearby, we find that up to 2% of the planetesimals
will fall either into planet-crossing orbits or into chaotic regions after the
mass-loss, depending on the location and mass of the planet (from Mars to
Neptune). This yields a sufficient mass of comets that can be scattered toward
the star, form a debris disk and pollute the atmosphere.
