Emission line central stars of planetary nebulae (CSPNe) often mimic the
spectra of massive WC stars, in spite of having undergone a completely
different evolutionary history, allowing Standard Model analyses to be
performed. One well known example is the low excitation star
CPD-56
8032, showing optical emission features due to
HeI-II, CII-IV and OII-III. This star has recently
been studied by Crowther et al. (1996) revealing it to have a dense,
stratified wind as demonstrated in Fig. 4, with a highly
evolved surface chemistry (Table 1). The wind temperature
of CPD-56
8032 obtained from the Standard Model is in excellent
agreement with that determined independently by De Marco et al. (1996),
validating the assumption of radiative equilibrium.
Overall, Wolf-Rayet type CSPNe (denoted [WC]) show a wider range in
stellar temperatures (20--140kK) than massive WC stars,
with luminosities and mass-loss rates 1--2 dex lower (Hamann 1996).
Chemical abundances are generally comparable with massive
WC stars, although hydrogen may be present in a subset of [WCL] stars.
Studies of [WC]-type CSPNe indicate close physical and chemical
similarities with PG 1159 stars. A potential evolutionary sequence is
[WCL]
[WCE]
PG 1159 stars (Hamann 1996).
Figure 4: Theoretical wind properties of the
[WC10] CSPNe CPD-56
8032 from Crowther et al. (1996),
demonstrating the stratification of W-R winds;
(a) Theoretical electron temperature distribution
(kK) as a function of Rosseland optical depth (
);
(b) electron density (in cm
);
(c)
fractional population for carbon; (d)
velocity distribution (10
kms
); (e)
radius scale (in R
); (f) line formation regions
for selected carbon lines