Investigation of the recombination of the retarded shell of "born-again" CSPNe by time-dependent radiative transfer models

TL;DR

This paper investigates the recombination processes in 'born-again' central stars of planetary nebulae using time-dependent radiative transfer models to understand the rapid changes in ionization states after late Helium flashes.

Contribution

It introduces a fully time-dependent radiative transfer model to study the ionization state evolution of CSPNe after late Helium flashes, addressing limitations of static models.

Findings

Recombination timescales can be significantly shorter than previously assumed.

Static models may not accurately describe the ionization state shortly after the flash.

Time-dependent models reveal rapid spectral changes in 'born-again' CSPNe.

Abstract

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.