Spectral analyses of late-type [WC] central stars of planetary nebulae: more empirical constraints for their evolutionary status
U. Leuenhagen1, W.-R. Hamann2
The optical spectra of the five recently observed late-type Wolf-Rayet central stars He 2-459 ([WC8]), M 2-43 ([WC8]), SWST 1 ([WC9]), PM 1-188 and IRAS 21282+5050 (both [WC11]) are analyzed by means of spherically expanding model atmospheres. The stellar parameters T* (effective temperature), vfinal (final velocity of the wind), R* (stellar radius) and dM/dt (mass loss rate) are determined by NLTE simulations which account for the elements hydrogen, helium, carbon and oxygen. With two exceptions (SWST and IRAS) the results presented here fit into the sample of already examined [WCL]-type objects. Altogether 13 out of 17 known [WCL]-CSPN have been analyzed so far. The presence of hydrogen in the atmospheres of [WC11] and [WC12] stars becomes more and more evident. In five out of seven analyzed objects of these subtypes hydrogen emission features of stellar origin can be identified. The spectra of the latest subtypes ([WC11], [WC12]) show rather narrow lines and thus allow to detect features of nitrogen (NII, NIII), neon (NeI) and silicon (SiIII, SiIV). For the first time we present model calculations accounting for these elements and perform abundance estimates for the eight narrow-lined stars (all [WC11] and [WC12] plus SWST). The obtained surface compositions are discussed in the light of recent evolutionary calculations which account for diffuse mixing during thermal pulses at the Asymptotic Giant Branch.