Interacting binary stars

The focus of our research to date has been on the investigation of semi-detached binary stars, principally the study of cataclysmic variables. Cataclysmic Variables (CVs) are close interacting binaries that contain a white dwarf accreting material transferred from a companion, usually a late main-sequence star. Unless the white dwarf has an extremely strong magnetic field, the transferred mass forms an accretion disk around this star. CVs are very active photometrically, exhibiting variability on time scales from seconds to centuries. An important reason to investigate the CVs is that they provide an unparalleled way to study two fundamental astrophysical processes: binary star evolution and nearly all aspects of the accretion of gas onto compact objects. Binaries are of fundamental importance to astronomers because they allow stellar masses, radii and luminosities to be measured directly and can also be used as distance indicators for nearby galaxies that are resolved into individual stars. In addition, accretion power – the extraction of energy from matter moving in a gravitational field – drives some of the most powerful energy sources in the Universe.

Credit: NASA/CXC/M.Weiss

Accretion disks can be found in a wide variety of environments, from the small disks around compact stars and protostars, to the massive disks around black holes in the centre of galaxies. The binary character of CVs together with their short orbital periods, and the rather small contribution from the stellar components to the produced optical radiation make these systems the ideal test beds for accretion disk theories. The disks in these systems are the dominant light source, they evolve on very short timescales (hours to weeks), and using various indirect imaging techniques, such as Doppler tomography, it is possible to probe structure in the disk with micro-arcsecond angular resolution. CVs are a vital link in the evolutionary chain of binary stars. Paradoxically, although the understanding of CVs and accretion disk physics has made much progress in recent years, the geometry of and physical conditions in these have remained highly uncertain. In fact, even a viable angular momentum transfer mechanism still remains the unresolved problem.