Planetary nebulae abundances and stellar evolution II
  Context: In recent years mid- and far infrared spectra of planetary nebulae
have been analysed and lead to more accurate abundances. It may be expected
that these better abundances lead to a better understanding of the evolution of
these objects. Aims: The observed abundances in planetary nebulae are compared
to those predicted by the models of Karakas (2003) in order to predict the
progenitor masses of the various PNe used. The morphology of the PNe is
included in the comparison. Since the central stars play an important role in
the evolution, it is expected that this comparison will yield additional
information about them. Methods: First the nitrogen/oxygen ratio is discussed
with relation to the helium/hydrogen ratio. The progenitor mass for each PNe
can be found by a comparison with the models of Karakas. Then the present
luminosity of the central stars is determined in two ways: first by computing
the central star effective temperature and radius, and second by computing the
nebular luminosity from the hydrogen and helium lines. This luminosity is also
a function of the initial mass so that these two values of initial mass can be
compared. Results: Six of the seven bipolar nebulae can be identified as
descendants of high mass stars (4Msun - 6Msun) while the seventh is ambiguous.
Most of the elliptical PNe have central stars which descend from low initial
mass stars, although there are a few caveats which are discussed. There is no
observational evidence for a higher mass for central stars which have a high
carbon/oxygen ratio. The evidence provided by the abundance comparison with the
models of Karakas is consistent with the HR diagram to which it is compared. In
the course of this discussion it is shown how `optically thin' nebulae can be
separated from those which are 'optically thick'.
