Two young PNe, NGC 7027 and BD+30 3639 were studied. Since dust extinction is expected to be important in these young objects, the optical observations were complemented with radio-interferometer data obtained with the MERLIN and VLA telescopes. The resolution in our radio maps is similar to that of the HST. Dust extinction was found to play an important role in NGC 7027 and less so in BD+30 3639. The interpretation of the optical spectroscopy is still in progress.
Hydrodynamic modelling of the formation of aspherical PNe including the effect of the evolution of central star showed that stars of different masses may produce nebulae of different morphology. The reason is that in the case of more massive central stars the fast increase in the UV flux produces a fast moving R-type ionization front which leaves the original aspherical density distribution of the circumstellar matter relatively undisturbed. In the case of lower mass stars the increase in UV photons is much more gradual and the ionization front is the slower D-type, which modifies the density distribution of the circumstellar material. This means that more massive central stars are more likely to produce more extreme bipolar nebulae.
In order to study flows in which photo-ionization and shocks both play a role in exciting the gas it is necessary to follow the ionization states of the different ions in a non-equilibrium fashion, taking into account both collisional and photo-ionization, radiative and dielectronic recombination, and charge exchange. Once the ionization states of the ions are known, the non-equilibrium cooling rates can also be calculated. A number of subroutines have been developed which can be combined with a hydrodynamic calculation. Using these routines for the first time the two-dimensional structure of a radiative bow-shock of a jet was calculated. The structure of a bow shock around a run-away O-star (in which case the photo-ionization is also important) was also modelled.
In an attempt to find an explanation for FLIERs, the pairs of bright
low-ionization knots seen in many PNe, the evolution of a neutral clump of
material exposed to ionizing radiation was modelled. Both a simplified
analytic description and a detailed numerical model (applying the realistic
ionization and cooling routines mentioned above) were used. The two models
agree remarkably well, and some of the properties of FLIERs, such as their
high [N II]/H 
ratio, can be explained. However, the HST images of
FLIERs show more complicated morphologies than can be reproduced with our
simple model. A similar model was also applied to study the evolution of
dense clumps being ionized by quasars, the objects responsible for the
Ly forest.