de Marco, Orsola; (1997) Cool Wolf-Rayet central stars and their planetary nebulae. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The research presented in this thesis is concerned with the properties of Wolf-Rayet central stars and their planetary nebulae (PN). Radial and nebular expansion velocities, reddenings and distances have been derived using different methods. A detailed analysis of the nebular physical parameters and abundances has been carried out for the PN of four cool Wolf-Rayet central stars, based on high resolution optical and UV spectroscopy. For the same sample, HST and ground based images are presented. Photoionization modelling of the PN is undertaken using different model stellar atmospheres and a comparison with empirically derived parameters is carried out. Non-LTE Wolf-Rayet stellar atmosphere modelling is described to the winds of two central stars with WC10 spectra. From this, stellar parameters are derived which are compared with estimates from independent model codes. The theory of low-temperature dielectronic recombination is applied and a method for the direct determination of the wind electron temperature is developed and applied to one WC11 and four WC10 central stars. Direct analysis of the stellar emission line spectrum of three of the four WC10 central stars is carried out using recombination line theory. A comparison is made between abundances derived using the non-LTE models and from recombination line analysis. Parameters derived from the non-LTE Wolf-Rayet models are used in an investigation of optical depth effects in the wind recombination lines. From this it is shown, for the first time, that low-lying lines usually considered to be optically thick and hence unsuitable for abundance determination, can be employed without substantial systematic effects. On the other hand, lines arising from dielectronic states are found to yield systematically lower abundances due to their being in LTE with the continuum. A classification system for Wolf-Rayet central stars is also developed, consistent with the classification of massive Wolf-Rayet stars.

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