Ebook: Cool Wolf-Rayet Central Stars and their Planetary Nebulae

The research presented in this thesis is concerned with the properties of Wolf-Rayet (WR) central stars and their planetary nebulae (PNe). The analysis was carried out on high (R=30,000) and intermediate (R=5,000) resolution optical and UV spectra of a sample of WR central stars and their PNe. The emphasis has been the determination of consistent stellar and nebular results via the use of a range of empirical and theoretical methods.

The definition of a robust classification system for central stars with WC spectra was proposed, consistent with the scheme of Smith et al. (1990, ApJ 358, 229) designed for massive WC stars. Our WC classification criteria have been thoroughly compared with past schemes and, in particular, WO and WCE classifications have been unified.

Nebular and stellar parameters and distances of the [WC10] central stars CPD-56 o 8032, He 2-113, M 4-18 and of the peculiar [WC9] SwSt 1 were determined. Distances were given a special consideration since they are the key to determining the range of luminosities of WR central stars. For instance, the difficulty in reconciling distances, luminosities and PNs characteristics for the spectroscopic twin [WC10] central stars He 2-113 and M 4-18, indicates the possibility of a slower evolution for M 4-18; this demonstrates that two identical WR central stars might follow different evolutionary paths.

From nebular analysis it has emerged that the most likely origin of the broad pedestal observed at the base of the Balmer profiles of CPD-56 o 8032, He 2-113 and M 4-18 is nebular rather than stellar emission as had been proposed by Leuenhagen et al. (1996, AA 312, 167). This reinstates WC central stars within their former hydrogen-free status so that the overlap between hydrogen-rich and hydrogen-poor central stars sequences is no longer obvious. Extremely high nebular C/H number ratios have been derived for CPD-56 o 8032 and He 2-113. Carbon enrichment can be expected in a variety if situations, however nebular C/H number ratios for M 4-18 and SwSt 1 are much lower showing that a high C/H ratio is not a prerogative of WR central stars. If carbon enrichment of the PN is at the hand of the stellar wind, the size and characteristics of the PN and the central star might explain the lack of carbon enrichment. Nebular parameters and abundances can be reproduced by photoionization modeling for the PNe of M 4-18 and SwSt 1 using WR model atmospheres, while modeling of CPD-56 o 8032 and He 2-113 was hampered by dust-gas competition in the ionized regions and no clear results could be obtained.

HST images, presented here for the first time, revealed that CPD-56 o 8032, He 2-113 and SwSt 1 are irregular and compact, while the PN of M 4-18 has a more developed morphology and, with a lower electron density than either of the other three, is certainly older, despite its spectrum being almost identical to that of He 2-113. No halo or second larger shell, which might reveal an association with the born-again scenario (Iben et al. 1983, ApJ 264, 605) is observed in these images.

Stellar recombination line analysis and non-LTE WR wind modeling (Hillier 1990, AA 231, 111) were used to determine stellar wind abundances and wind parameters. The use of dielectronic recombination lines was used for the determination of the wind electron temperature in the C ii line forming region of 5 [WCL] stars (17,000 to 20,000 K). By comparison with the electron temperatures predicted by the wind models, this constitutes the first confirmation of the assumption of radiative equilibrium in the winds of WR stars. Optical depth effects on C ii bound-bound recombination lines were shown not to preclude their use for abundance determination.

It is argued that evidence is accumulating towards the fact that there are different evolutionary sequence for all post-AGB WC central stars of PN, in the sense that it does not appear that the [WC] stars are either all post-AGB or all created though a born-again scenario.