Ionization-bounded and Density-bounded Planetary Nebulae

Planetary nebulae (and in general any photoionized region) can be classified as ionization-bounded or density-bounded. It is important to determine in which case is the planetary nebula studied to be able to estimate from nebular observations, for example, the total rate of ionizing photons produced by the central star. In this paper we present a simple observational criterion that uses radio continuum images and that allows to establish a necessary (but not sufficient) condition for the planetary nebula to be considered as ionization-bounded. We apply the criterion to two planetary nebulae: NGC 7027 is most possibly ionization-bounded, while Hb~4 is density-bounded, at least in some directions.