Investigation of the recombination of the retarded shell of "born-again"
  CSPNe by time-dependent radiative transfer models
  A standard planetary nebula stays more than 10 000 years in the state of a
photoionized nebula. As long as the timescales of the most important ionizing
processes are much smaller, the ionization state can be characterized by a
static photoionization model and simulated with codes like CLOUDY (Ferland et
al. 1998). When the star exhibits a late Helium flash, however, its ionizing
flux stops within a very short period. The star then re-appears from itsopaque
shell after a few years (or centuries) as a cold giant star without any hard
ionizing photons. Describing the physics of such behavior requires a fully
time-dependent radiative transfer model. Pollacco (1999), Kerber et al. (1999)
and Lechner & Kimeswenger (2004) used data of the old nebulae around V605 Aql
and V4334 Sgr to derive a model of the pre-outburst state of the CSPN in a
static model. Their argument was the long recombination time scale for such
thin media. With regard to these models Schoenberner (2008) critically raised
the question whether a significant change in the ionization state (and thus the
spectrum) has to be expected after a time of up to 80 years, and whether static
models are applicable at all.
