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Central stars of Planetary Nebulae

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

Paul Crowther
Mon Feb 12 13:51:35 GMT 1996