Wolf-Rayet stars

Three sources have been proposed to produce fluorine in the Galaxy. The first was suggested by Forestini et al. (1992) and refers to production in low-mass stars during the AGB phase while two others are related to massive stars production in Wolf-Rayet stars (Meynet Arnould 2000) and in type II Supernovae, via the neutrino-induced nucleosynthesis (Woosley et al. 1990). 

The rotation models predict significant effects on the properties and the evolution of the massive stars. They alter the ratio of red to blue supergiants and hence the nature of SNII progenitors they affect the properties, formation and evolution of Wolf-Rayet stars they result in the enrichment of He and C in the ISM while the abundance of O decreases they produce higher He and a-element yields from SNII via larger He cores. Many of these effects are metallicity dependent. With such far ranging impact, the effects of rotation and mass loss on the evolution of massive stars should be thoroughly understood. 

For Wolf-Rayet stars of type WN, rotation makes smoother changes of abundances, due to internal mixing. For WN stars, the transition phase, with still H present, becomes longer due to rotation and this increases the late WN phase (WNL) where H is usually present. The CNO abundances at the end of the WN phase are the same for rotating and non-rotating models, because they are model independent and determined just by CNO nuclear equilibrium. Indeed, CNO abundances in WN stars provide a unique test of the physics of the CNO cycle. 

R. F. Kudritsky and D. G. Hummer, Quantitative Spectroscopy of Hot Stars , Ann. Rev. Astr. Astrophys., 28, 303, 1990 W. Schmutz, C. Leitherer and R. B. Gru-enwald, Theoretical Energy Distributions of Wolf-Rayet Stars , Pub. Astr. Soc. Pacific, 104, 1164, 1992 A.W. A. Pauldrach, T.L. Hoffmann and M. Lennon, Radiation-driven winds of hot luminous stars. XIII A description of NLTE line blocking and blanketing towards realistic models for expanding atmospheres , A ... 

A Y/ A Z or AF/A(0/H) may vary, either systematically as a function of Z or randomly, e.g. if some H n regions are self-polluted with helium and other elements (e.g. N) ejected in winds from massive embedded stars. There is some evidence that this actually happens in a very few cases, such as the nucleus of NGC 5253, where the continuum shows a strong, broad feature at the He+ wavelength X 4686, due to Wolf-Rayet stars. 

Fig. 5.13. Time evolution of the chemical profile of a 40 Mq star that becomes a Wolf-Rayet star as a result of the outer layers peeling off in stellar winds. The spectrum evolves from type O to type B to a red supergiant (RSG) and then back to a blue supergiant (BSG) and towards increasing effective temperatures ending up well to the left of the main sequence. The chemically modified spectrum evolves from nitrogen-rich late, i.e. relatively cool (WNL), to nitrogen-rich early (WNE) to carbon-rich (WC) in some cases still hotter stars are observed that are oxygen-rich (WO). After Maeder and Meynet (1987).

Massive stars from 25 to 100 Mo already lose a substantial fraction of their mass in strong stellar winds ranging from 2x10 Mo/y during the H-Buming phase up to 5 x 10 Mo/y in the He-buming phase also known as Wolf-Rayet phase. As a large convective core develops in these stars, fresh nucleosynthetic product are soon exposed on the surface and ejected with the stellar winds (Prantzos et al. 1986). Wolf-Rayet stars may have been the principal source of Al in meteorites (Amould et al. 2000). 

Wolf-Rayet star massive star producing a high stellar wind... 

The different types of grain can be related to specific classes of stellar objects. The very hot and bright, even lavish Wolf-Rayet stars are considered to be one of the most favourable sites for grain formation, for their strong stellar winds are particularly rich in carbon. Matter thrown out by supernovas and cooling very quickly due to its expansion is also an excellent scenario for grain formation. Elements with any affinity for the solid state are likely to be abundantly transformed. 

Fig. 4.4. All-sky map in the light of the 1.809 MeV gamma-ray hne from radioactive aluminium-26. The galactic distribution of aluminium-26, based on data from the COMPTEL (Compton Telescope) experiment aboard the GRO (Gamma-Ray Observatory), suggests that this isotope is dispersed across the Galaxy by the most massive stars, Wolf-Rayet stars and supernovas. Al is formed by the reaction Mg -b p — A1 -b y. This radioactive isotope has a lifetime of about million years and is ejected into space before it begins to decay.

Wolf-Rayet stars SNIa SNIb/c SNII Nova... 

Wolf-Rayet stars as a diagnostic of internal mixing processes in massive mass losing stars... 

A systematic study of stellar models for C/O-rich Wolf-Rayet stars... 

It is remarkable that He-rich stars appear to have a higher rate of mass loss than stars with solar-type atmospheres with the same Teff and L-values The Wolf-Rayet stars have, on the average, Al-values that are 140 times larger than the values for corresponding 0 and B type stars (De Jager et al., 1987). This may be due to the fact that WR stars, with their large Helium abundance, are relatively closer to their Eddington limit than the most luminous 0-type stars. 

The Wolf-Rayet classification is a one-dimensional system. However, as outlined in Sect. 4, their spectra depend on two parameters. Therefore, a two-dimensional classification clearly has to be introduced, e.g. as already suggested by Hiltner and Schild (1966) and refined by Walborn (1974). For later use in this paper, we divide the Wolf-Rayet stars into four classes ... 

Approximate analyses as described in Sect. 6.1. have been performed for 30 galactic Wolf-Rayet stars. The resulting stellar parameters T, L and M are given graphically in the Figure 1 of Schmutz et al. (1987c, these proceedings). In the same... 

If Indeed only a minority of the Wolf-Rayet stars were in the post-red-supergiant phase, the consequences would be twofold First, the estimate of the ratio of O-star to Wolf-Rayet life-tines, tfl/tWR, has to be revised, and second, all the indirect ethods used to derive the initial nasses of the Wolf-Rayet stars fron the age of their surrounding would be not valid (Conti et al., 1983 Schild and Maeder, 1984). 

Temperatures, mass-loss rates and luminosities of 30 galactic Wolf-Rayet stars (24 WN, 6 WC) are derived by fitting the observed equivalent widths of He I A5876 and He II A5412 and the absolute visual magnitude. A three-dimensional grid (T -R -M) of model calculations provides the theoretical values. 

It is obvious that the derived parameters are not in agreement with the "standard" evolutionary calculations leading to Wolf-Rayet stars (Maeder and Meynet, 1987). The most extreme disagreement is found for the WC stars. 

Figure 1. The location in the HR-diagram of the 30 Wolf-Rayet stars as resulting from this work. The effective temperatures are referred to the core" radii...

NLTE ANALYSIS OF THE WOLF-RAYET STAR HD193077 (WN5+abs)... 

A model atmosphere code that accounts for the special physical conditions in Wolf-Rayet atmospheres (Hamann and Schmutz, 1986 Wessolowski et al., 1987) is used to analyse the spectrum of the Wolf-Rayet star HD193077 (WN5+abs). The stellar parameters are determined such that the profiles of the helium lines He I 4471, 5876, He II >5412, and the absolute visual magnitude are reproduced. 

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