The rebrightening of planetary nebulae through ISM interaction
  The interaction of planetary nebulae (PNe) with the interstellar medium as
they move through it is now acknowledged to be a major shaping effect not just
for ancient and large PNe, but also for relatively young PNe with high speed
central stars. The most common effect is a rebrightening as the PN shell
interacts with a pre-existing bow shock structure formed during the previous
evolutionary phase of the central star. In this review, we consider this
rebrightening in detail for the first time and discuss its origins,
highlighting some observed examples. We go on to discuss the AGB star
progenitors, reviewing the evidence for bow shock structures, and consider the
progeny of rebrightened PNe - strongly disrupted objects which bear very little
resemblance to typical PNe. Sh 2-68 is inferred to be perhaps the only
documented case so far of such a PN.
