Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10 6 L and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L ) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L ) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L ) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M . This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.

Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium-and carbonenriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected.

Abstract. X-ray observations of young Planetary Nebulae (PNe) have revealed diffuse emission in extended regions around both H-rich and H-deficient central stars. In order to also reproduce physical properties of H-deficient objects, we have, at first, extended our time-dependent radiation-hydrodynamic models with heat conduction for such conditions. Here we present some of the important physical concepts, which determine how and when a hot wind-blown bubble forms. In this study we have had to consider the, largely unknown, evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind, and the chemical composition. The main conclusion of our work is that heat conduction is needed to explain X-ray properties of wind-blown bubbles also in H-deficient objects. Keywords. conduction, hydrodynamics, planetary nebulae: generalIn Steffen et al. (2008) we present time-dependent radiation-hydrodynamic models of PNe that include heat conduction at a solar (H-rich) composition. Extending the models to work with H-deficient compositions, we note that the general Fokker-Planck-based plasma theory of Spitzer (1962), to good accuracy, applies to both pure hydrogen and other chemical compositions. Heat conduction is modeled through a heat-flux term q, which is written as a diffusion coefficient (D) multiplied with the temperature gradient, q = −D∇T . The charge-dependent factor in D -i.e. εδ T Z −1 , where ε(Z) and δ T (Z) are given in Spitzer & Härm (1953) -decreases modestly with increasing effective charge Z. Our more general tests with models that use typical H-deficient abundances show that Z 4.0; the highest values are reached inside the hot wind-blown bubble in the PNe. Figure 1a shows that D(Z), under H-deficient conditions, it is about a factor two smaller than in a pure hydrogen plasma of the same temperature, a rather insignificant decrease.The treatment of the stellar evolution is a crucial issue in time-dependent models of the ionization structure in PNe. The quality of the models increases with more accurate predictions of the history of the slow wind (of the previous asymptotic-giant-branch stage), the fast wind, the central star (CSPN), and the abundances. The winds are characterized by their mass-loss rate, outflow velocity, and abundances; and the CSPN by its effective temperature (T eff ), luminosity, and mass. For H-rich compositions Pauldrach et al. (1988Pauldrach et al. ( ,2004 find that the mass-loss rate of the fast wind decreases with time.

With the help of detailed nebula modeling and X-ray observations, we want to shed light on the enigmatic origin of Wolf-Rayet type central stars of planetary nebulae. This method allows us to assign observed [WC] stars to one of the proposed evolutionary scenarios, attributing the loss of hydrogen to a "late", "very late", or an "AGB final" thermal pulse (LTP, VLTP and AFTP, respectively). Following our analysis, we conclude that BD +30 • 3639 evolved through an AFTP.