Accretion Disks in Close Binaries

Abstract

With the improvement of observational techniques in astronomy the importance of accretion processes in objects of quite different nature, ranging from double stars in a close orbit to active nuclei of galaxies, was recognized. Wherever a surplus of angular momentum keeps mass from falling directly onto a central object an accretion disk might be formed. This can happen around single objects (for example in connection with star formation or probably in active galactic nuclei) as well as in binary systems. Mass transfer in close binaries is a classical example. There accretion disks can occur in systems of various combinations of main-sequence stars, evolved stars or compact stars as primary and secondary component in a close orbit. The accretion onto a compact star is especially important because the accretion luminosity is proportional to the inverse of the radius of the mass-gaining star. For a neutron star the release of gravitational energy from accreted matter per unit mass is much higher than the release of energy from nuclear burning of the same amount of hydrogen to helium and heavier elements. Thus, accretion disks can be very bright. They are, therefore, very important for our understanding of the physics displayed in binary systems.