Evolution of Close Binaries

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The evolution of a star depends primarily on its mass, chemical composition, so it is important to know the distribution of stellar masses when the stars are born. Stars can be born as singles, in pairs, or in multiple systems. A large fraction of the stars belongs to binary systems. Hence it is essential to know how stellar masses are distributed at birth, how many stars are formed, not only as singles, but also what fraction originates as binary systems, and what is then the mass ratio and orbital period distribution of these binaries. Since massive stars are intrinsically the brightest ones, it is interesting to try to discover their properties in distant stellar groups. Until now observations of massive stars are reasonably complete for a restricted portion of our own Galaxy, within a sphere with a radius of about 3 kpc from the Sun. Conclusions from these observations are then extended to the whole Galaxy. Before starting the study of evolution of close binary systems, we examine rst the evolution of single stars. So we begin to deal with the stellar structure processes, stellar evolution in general, we investigate how evolution occurs dierently for stars of low, intermediate and large mass. Comparison of evolutionary computations with observations allows restricting a number of uncertainties of the value of parameters in the equations of stellar structure and evolution. The evolution of close binaries diers from the evolution of single stars, since the presence of a companion sets limits on the stellar expansion during its evolution. We examine the processes that have to be added to the stellar structure equations to compute the structure of the components of binary systems, and to calculate their evolution. Here also we study in detail the uncertainties and their eects on the results of the computations. An overview is presented of the existing evolutionary computations for various masses, with various assumptions on the processes, stellar wind mass losses for massive components, convection criteria, the behaviour of semiconvection, the accretion process, standard accretion models, full mixing models, the formation of common envelope systems, the formation of discs. Comparison of computations with observations of binaries that are representative for a given class allows to draw general conclusions and to discard a number of uncertainties. The combination of the results of massive single stars and massive binaries allows predictions concerning the evolution of certain stellar populations ( Wolf-Rayet stars, luminous blue variables, X-ray binaries, ...), i.e. predictions on their progenitors, and their evolutionary history. We investigate uncertainties in binary statistics (frequency, mass ratio and period distribution) and try to determine how these uncertainties aect the results. Observations of massive single stars and of massive close binaries, allow to present a consistent evolutionary model for both, that is able to explain the stellar content and the overall stellar properties for the solar neighbourhood.