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.
