Evolutionary Processes in Close Binary Systems

The structure and evolution of the stars constitute one of the main prob­ lems of modern astrophysics. Binary stars are so common that it is not pos­ sible to develop a comprehensive theory of stellar structure and evolution without taking into account the duplicity of so many objects. According to }aschek & Gomez (1970) 50% o f the main-sequence stars are binaries. Van Albada & Blaauw (1967) estimated that 60% of the early-type stars are double with mass ratio larger than 0.2. Some types of objects are found in binary systems only: metallic-line stars (Abt 1961, 1965, Abt & Bidelman 1969), U Germinorum variables and novae (Kraft 1962, 1963, 1964), a large fraction of Wolf-Rayet stars (Underhill 1968). Hoyle (1964) and McCrea (1964) suggested that blue stragglers in open and globular clusters are the products of mass exchange in close binaries. This idea found some observa­ tional support in the case of open clusters (Cannon 1968, Deutch 1969, Strom & Strom 1970). Blaauw (1961) suggested that the early-type run­ away stars had escaped from close binaries disrupted as a result of a super­ nova explosion of another component. Binaries are considered to be possible candidates for X-ray sources (Shklovsky 1967, Cameron & Mock 1967, Prendergast & Burbidge 1968). The general evolution of Population I objects is strongly affected by the commonness of binaries within this population. It should be emphasized that duplicity is not found among the Population II stars . I t is also remarkable that none of the 50 known pulsars is in a binary. The most fundamental physical parameter of a star , its mass, may be reliably measured for components of binary systems only. Eclipsing variables allow us to study in some detail the surface of their components. Distribution of mass in a stellar envelope affects the orbital motion and leads to apsidal motion. This phenomenon makes it possible to check the structure of stellar models. It is generally accepted now that mass transfer in a binary from one component to another is the most basic phenomenon in the evolution of such a system. Therefore matteJ1 with a chemical composition modified by nuclear reactions may be expected to be lifted up to the stellar surface. The two out­ standing examples are the binaries HD 30353 and v Sgr (Wallerstein 1968), deficient in hydrogen and overabundant in nitrogen. The matter that is visi-

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