ANALYSES OF PNNI WITH [WC] SPECTRAL TYPE

A number of Planetary Nebula Nuclei (PNNi) exhibit emission-line spect ra resembling those of Population I Wolf-Rayet stars of the carbon seq uence (WC). As these kinds of emission-line spectra are formed by expa nding stellar atmospheres (stellar winds), their analysis requires spe cial non-LTE model calculations which we have developed in Kiel. The [ WC]-type PNNi form two clearly separated groups. One group termed as ' 'early'' subtypes ([WCE]) consists of very hot stars. So far we have a nalyzed four representatives (Abell 30, Abell 78, Longmore 4, NGC 6751 ) obtaining effective temperatures above 100 kK. Their chemical compos ition (predominantly helium, carbon and oxygen) reveals their close re lationship to the very hot white-dwarf progenitors called PG 1159 star s, the only difference to the latter being the higher mass-loss rate o f the [WCE] stars. In one star (Longmore4) a transient change of the s pectral appearance between these two types was observed fortuitously. The second group is of ''late'' spectral subtypes ([WCL]) and comprise s relatively cool stars. The representative which we have analyzed so far (V 348 Sgr) shows a very strange composition, as hydrogen is prese nt simultaneously with enriched carbon. The low effective temperature of that star (20 kK) cannot sustain the ionization of its nebula, lead ing to the hypothesis of a very rapid phase of evolution.