A revision of the theory of hot-star winds

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Some stars are formed with ten or even hundred times more mass than the sun. Such "massive" stars play an important role in the cosmos. Their radiation is up to million times stronger than that of the sun, and they drive a strong stellar wind by which they disperse the major part of their mass during their short lifetime. Finally they explode as supernova. The ubiquity and importance of mass loss from hot stars has been discovered almost 50 years ago. Soon after, a theory was proposed in order to explain how such huge amount of matter can de blown off. The essential mechanism is the pressure that is exerted by the enormously intense radiation of the star. The matter in the stellar wind partly absorbs this radiation and is thus accelerated. The mentioned theory of radiation-driven winds describes these very complicate processes under drastic approximations. Nevertheless, the results were in good agreement with observations, at least for those types of stars which were studied first (supergiants of spectral types O and B). From today's perspective, this good agreement was perhaps only by incidence. In the meantime, the models for analyzing the observations have been improved and now imply that the most important parameter, the mass-loss rates, have to be revised downward significantly. For other types of massive stars, the mentioned theory was not applicable anyhow, or gave wrong predictions. Various authors have already challenged one or another of the approximations on which the mentioned stellar-wind theory is based on - partly with alarming results. The detailed treatment of the radiation processes in stellar winds requires an enormous effort. However, such efforts are unavoidable for the interpretation and quantitative analysis of the spectra from hot stars with winds. The Potsdam group has developed a corresponding computer code (Potsdam Wolf-Rayet model atmospheres - PoWR). Only very few comparable codes exist worldwide. These simulations of stellar spectra can also evaluate the radiation pressure at any point in the atmosphere. On this basis, we have developed a method to model the hydrodynamics of the stellar wind consistently. The drastic approximations of the stellar wind theory mentioned above are thus avoided. Our models explain for the first time the winds of the so-called Wolf-Rayet stars, which are the strongest stellar winds observed.In the new project, these calculations shall now be applied to stellar winds in general. One needs to understand, where and why the different approximations in the schematic stellar-wind theory break down. After the new models have been checked with observations, we will perform hydrodynamic simulations for the widest variety of stars and predict their mass-loss rates. Such predictions are urgently needed in order to understand the evolution of massive stars, stellar clusters, and whole galaxies.