SIMBAD references

Density distributions and velocity fields are derived for four spatially resolved planetary nebulae, based on high-resolution long-slit emission line spectra. We use a photo-ionization code to fit the spectra and to derive the nebular structure, assuming spherical symmetry. We present the results for individual nebulae and compare with three objects analyzed earlier. The expansion velocities are found to increase outward: this is shown to be likely related to the presence of an ionization front. Six PN in the sample are found to be ionization bounded. For one object, which has a WR-type central star, a large amount of turbulence or irregular structure in the velocity field is found. The results are in agreement with hydrodynamical calculations, in which the velocities also tend to increase with distance from the star. However, the common presence of an ionization front was not predicted.