Molecular Photodissociation in the Neutral Envelopesof Planetary Nebulae
  The dissociation and subsequent ionisation of molecular material in a detached, spherically symmetric stellar wind is modelled as a star evolves from the post-asymptotic giant branch phase to a planetary nebula (i.e. from a stellar effective temperature of Teff=6000K to Teff=33000K). The model is compared to the observed mass and distribution of ionised and atomic hydrogen in the planetary nebula IC418. It is found that the mass of circumnebular atomic hydrogen observed in IC418 can easily be produced by the dissociation of originally molecular material by the ultraviolet radiation of the central star. However, the observed spatial distribution of the gas is not accurately recreated by the current model. 
G.T.Gussie, A.R.Taylor, P.E.Dewdney, and R.S.Roger