Classical Wolf-Rayet (WR) stars mark an important stage in the late evolution of massive stars. As hydrogen-poor massive stars, these objects have lost their outer layers, while still losing further mass through strong winds indicated by their prominent emission line spectra. Wolf-Rayet stars have been detected in a variety of different galaxies. Their strong winds are a major ingredient of stellar evolution and population synthesis models. Yet, a coherent theoretical picture of their strong mass-loss is only starting to emerge. In particular, the occurrence of WR stars as a function of metallicity (Z) is still far from being understood.

To uncover the nature of the complex and dense winds of Wolf-Rayet stars, we employ a new generation of model atmospheres including a consistent solution of the wind hydrodynamics in an expanding non-LTE situation. With this technique, we can dissect the ingredients driving the wind and predict the resulting mass-loss for hydrogen-depleted massive stars. Our modelling efforts reveal a complex picture with strong, non-linear dependencies on the luminosity-to-mass ratio and Z with a steep, but not totally abrupt onset for WR-type winds in helium stars. With our findings, we provide a theoretical motivation for a population of helium stars at low Z, which cannot be detected via WR-type spectral features. Our study of massive He-star atmosphere models yields the very first mass-loss recipe derived from first principles in this regime. Implementing our first findings in stellar evolution models, we demonstrate how traditional approaches tend to overpredict WR-type mass loss in the young Universe.

The recent discovery of a gravitational wave from the merging of two black holes of about 30 solar masses each challenges our incomplete understanding of massive stars and their evolution. Critical ingredients comprise mass-loss, rotation, magnetic fields, internal mixing, and mass transfer in close binary systems. The imperfect knowledge of these factors implies large uncertainties for models of stellar populations and their feedback. In this contribution we summarize our empirical studies of Wolf-Rayet populations at different metallicities by means of modern non-LTE stellar atmosphere models, and confront these results with the predictions of stellar evolution models. At the metallicity of our Galaxy, stellar winds are probably too strong to leave remnant masses as high as ~30 M⊙, but given the still poor agreement between evolutionary tracks and observation even this conclusion is debatable. At the low metallicity of the Small Magellanic Cloud, all WN stars which are (at least now) single are consistent with evolving quasi-homogeneously. O and B-type stars, in contrast, seem to comply with standard evolutionary models without strong internal mixing. Close binaries which avoided early merging could evolve quasi-homogeneously and lead to close compact remnants of relatively high masses that merge within a Hubble time.

We present XMM-Newton and Chandra observations of the born-again planetary nebula A 30. These X-ray observations reveal a bright unresolved source at the position of the central star whose X-ray luminosity exceeds by far the model expectations for photospheric emission and for shocks within the stellar wind. We suggest that a “born-again hot bubble” may be responsible for this X-ray emission. Diffuse X-ray emission associated with the petal-like features and one of the H-poor knots seen in the optical is also found. The weakened emission of carbon lines in the spectrum of the diffuse emission can be interpreted as the dilution of stellar wind by mass-loading or as the detection of material ejected during a very late thermal pulse.

To understand the evolution and morphology of planetary nebulae, a detailed knowledge of their central stars is required. Central stars that exhibit emission lines in their spectra, indicating stellar mass-loss allow to study the evolution of planetary nebulae in action. Emission line central stars constitute about 10 % of all central stars. Half of them are practically hydrogen-free Wolf-Rayet type central stars of the carbon sequence, [WC], that show strong emission lines of carbon and oxygen in their spectra. In this contribution we address the weak emission-lines central stars (wels). These stars are poorly analyzed and their hydrogen content is mostly unknown. We obtained optical spectra, that include the important Balmer lines of hydrogen, for four weak emission line central stars. We present the results of our analysis, provide spectral classification and discuss possible explanations for their formation and evolution.

X-ray observations of young Planetary Nebulæ (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.

Massive hot stars are strong sources of X-ray emission originating in their winds. Although hydrodynamical wind simulations that are able to predict this X-ray emission are available, the inclusion of X-rays in stationary wind models is usually based on crude approximations. To improve this, we use results from time-dependent hydrodynamical simulations of the line-driven wind instability to derive an analytical approximation of X-ray emission in the stellar wind. We use this approximation in our non-LTE wind models and find that an improved inclusion of X-rays leads to a better agreement between model ionization fractions and those derived from observations. Furthermore, the slope of the Lx-L relation is in better agreement with observations, albeit the X-ray luminosity is underestimated by a factor of three. We propose that a possible solution for this discrepancy is connected with the wind porosity.

We present a parameter study of WR-type mass loss, based on the PoWR hydrodynamic model atmospheres. These new models imply that optically thick WR-type winds are generally formed close to the Eddington limit. This is demonstrated for the case of hydrogen rich WNL stars, which turn out to be extremely massive, luminous stars with progenitor masses above ≈ 80 M⊙. We investigate the dependence of WR-type mass loss on various stellar parameters, including the metallicity Z. The results depend strongly on the L/M ratio, the stellar temperature T*, and the assumed wind clumping. For high L/M ratios, strong WR-type winds can be maintained down to very low Z. Even for primordial massive stars we predict considerable mass loss if their surfaces are self-enriched by primary elements.

We present recent results for galactic WNL stars, obtained with the new Potsdam Wolf-Rayet (PoWR) hydrodynamic model atmospheres. Based on a combination of stellar wind modeling and spectral analysis we identify the galactic WNL subtypes as a group of extremely luminous stars close to the Eddington limit. Their luminosities imply progenitor masses around 120 M⊙ or even above, making them the direct descendants of the most massive stars in the galaxy. Because of the proximity to the Eddington limit our models are very sensitive to the L/M ratio, thus allowing for a direct estimate of the present masses of these objects.

According to previous spectral analyses of Wolf-Rayet type central stars, late [WC] subtypes show systematically higher carbon-to-helium abundance ratios than early [WC] subtypes. If this were true, it would rule out that these stars form an evolutionary sequence. However, due to the different parameter domains and diagnostic lines, one might suspect systematic errors being the source of this discrepancy. In an ongoing project we are therefore checking the [WC] analyses by means of the last generation of non-LTE models for expanding stellar atmospheres which account for line-blanketing and wind clumping. So far, the abundance discrepancy is not resolved. Further element abundances (H, N, Fe) are determined and compared with evolutionary predictions.

The Potsdam non-LTE code for expanding atmospheres, which accounts for clumping and iron-line blanketing, has been used to establish a grid of model atmospheres for WC stars. A parameter degeneracy is discovered for early-type WC models which do not depend on the ‘stellar temperature’. 15 Galactic WC4-7 stars are analyzed, showing a very uniform carbon abundance (He:C = 55:40) with only few exceptions.

The blue compact H ii galaxy CTS 1026 shows very strong WR emission features (Wλ(4680) = 17 Å and Wλ(5800) = 10 Å). We present high s/n optical spectra of the nucleus of this object, augmented by IUE UV spectra. By analysis of the WR profile shapes, we determine the dominant spectral types and the number of WN and WC stars in the star forming region. The number of O-type stars is determined by comparing the observed UV to optical flux distribution to population synthesis models.

Wolf-Rayet type central stars have been analyzed with adequate model atmospheres. The obtained stellar parameters and chemical abundances allow for a discussion of their evolutionary origin.

We discuss the connection between wind overloading and discrete absorption components in P-Cygni line profiles from O-type stars. Overloading can create horizontal plateaus in the radial wind speed that cause the extra absorption in the line profile. The upstream propagation speed of these velocity plateaus is analyzed. The second part of the paper deals with X-ray emission from O-type stars. X-ray line profiles observed with Chandra and XMM-Newton are often symmetric, contrary to what is expected for lines from a homogeneous wind. We discuss the influence on line symmetry of photon escape channels in a strongly clumped wind.

Absorbing material compressed in a number of thin shells is effectively less opaque for X-rays than smoothly distributed gas. The calculated X-ray emission-line profiles are red-shifted if the emission arises from the starward side of the shells.

Recent non-LTE models for expanding atmospheres, accounting for iron group line-blanketing and clumping, show a radiative acceleration which supplies a large part of the driving force of WR and O star winds. Aiming at the calculation of fully consistent wind models, we developed a method to include the solution of the hydrodynamic equations into our atmosphere code, taking into account the radiation pressure from the CMF radiation transport. In the present work we discuss the resulting wind acceleration for ‘standard’ WR and O-type star model atmospheres, and present a hydrodynamically consistent non-LTE model for the O4I(n)f star ζ Pup. In addition we demonstrate the effect of clumping on the radiative acceleration.

Spectra of seven WC stars in the LMC are taken with the HST-FOS. They are analyzed by means of non-LTE models for spherically expanding atmospheres with complex model atoms of helium, carbon and oxygen taken into account. We find stellar luminosities in the range from 1051–56 L⊙. The stellar temperature T*, defined as the effective temperature related to the stellar core radius, is of the order of 100 kK. The atmospheric compositions show carbon mass fractions of about 0.4 and oxygen mass fractions in the range 0.1–0.5.

By progress in both computer-speed and the development of efficient solution algorithms (Accelerated Lambda Iteration, ALI) nowadays detailed models for Wolf-Rayet atmospheres can be calculated on a desktop computer. In recent years much work was focused on the correct treatment of all relevant opacities, with the success that now hundreds of atomic levels and thousands of spectral lines of light elements (e.g., He, C, N, O) and millions of Fe lines can be accounted for in non-LTE. With respect to the opacities, these new models are complete but they are all based on the standard assumptions for WR atmospheres, namely spherically-symmetric expansion and stationarity and homogeneity of the outflow (cf. Hamann et al. these Proceedings).

The quantitative analysis of Wolf-Rayet type spectra requires non-LTE models for expanding atmospheres which have been developed in the last decade. Idealizing assumptions of the so-called standard model are spherical symmetry, homogeneity and stationarity. While the first generations of models were restricted to simple atoms (helium, hydrogen), we now apply more advanced calculations which account for complex model atoms (nitrogen, carbon, oxygen). The re-analysis of the Galactic WN sample from the nitrogen lines led to a substantial revision of the stellar parameters, as far as the WNE-w subclass is concerned. Wind inhomogeneity ('clumping') has been accounted for in a first-order approximation. The degree of clumping is derived from the electron-scattering wings of strong lines. With clumped models, considerably lower mass-loss rates (by a factor two or more) are obtained, while other empirical parameters are not affected significantly. An overview is given about the fundamental parameters of Wolf-Rayet stars as obtained from spectral analyses, including new results on WN stars in the LMC.

We have modeled the spectrum of WR135 with a co-moving frame code and then used identical input parameters with a Sobolev-approximation code. Helium lines compare well in strength and shape, while carbon lines are found to be much stronger in the co-moving frame code. The reason for this discrepancy is being investigated. The present comparison shows that tests have to be extended to more complex atomic models and compatibility between different codes should not be taken yet for granted.

We have carried out a full spectroscopic analysis of the WC8 star in the γ2 Vel binary (WR11). Through the binary radial-velocity curve the mass of the WR star can be determined, and hence its luminosity can be derived through the mass-luminosity relation for WR stars. This can be compared to the luminosity from the spectroscopic analysis. We find that the standard modeling methods underestimate the luminosity of the WR star.