Abstract
There has been a considerable level of uncertainty as to whether planetary nebulae (PNe) contain radial density gradients. Whilst the work of Taylor et al. (1987) implies that gradients exist in most PNe, that of Siódmiak & Tylenda (2001) suggests precisely the reverse. The work of Phillips (2007) suggests that both of these analyses are suspect, however, and that at least ∼ 10 → 20% of PNe probably do contain gradients. We now extend this latter analysis to include significantly higher frequency fluxes, and use these to undertake a more sensitive analysis of the effects of radial density gradients. We conclude than in excess of 85% of PNe appear to contain gradients, of which ∼ 21% are likely to have density exponents β > 1.5, and central cavity sizes which are small (i.e. have radii < 20% of the outer radius of the shell).
