[] Compact Binaries

The study of binary stars is fundamental to all areas of astrophysics. Most stars reside in binaries; measurements of the dynamical interaction of the components provide the most accurately determined parameters of stars. Exotic interacting binaries with collapsed stars are among the most powerful energy sources in the Universe, outshining their host galaxies. Interacting binaries are a rich laboratory for a wide variety of astrophysical phenomena.

Particularly promising is the study of compact binaries, interacting double stars where a normal star transfers mass to a compact object, a stellar corpse, the collapsed endstate of stellar evolution. The transferred matter heats up dramatically as it funnels down the compact object's potential well, causing the emission of high-energy radiation. Compact binaries with a neutron star or black hole (X-ray binaries) are the most powerful X-ray sources in the Galaxy, while white dwarf systems, (cataclysmic variables, CVs) are easily recognised from the distinct time-variability in the optical waveband.

These endstates of stellar evolution hide a wealth of information on the entire previous stellar lifecycle, and on the physical processes governing this evolution.

The impact of compact binary research is threefold.

  1. Compact binaries are the cleanest high-energy laboratories for the study of accretion, which plays a vital role in planet and star formation, and in powering active galaxies.
  2. Compact binaries provide stringent tests of stellar models and hence of the physics of radiation and matter in stellar plasmas. The presence of a companion star allows one to constrain parameters of either component in a quality usually impossible for single stars.
  3. A theoretical understanding of the statistical properties of compact binary populations in different stellar ensembles allows one to exploit observations of high-energy events as diverse as X-ray novae, gamma-ray bursts, type Ia supernovae, and pulses of gravitational wave emission, from galaxies other than the Milky Way.

Information on compact binary formation and evolution - essential for any attempt to understand processes observed in individual systems - can be deduced by extracting common properties and characteristic system parameter distributions from the wealth of observational data. We apply the population synthesis technique to results from systematic X-ray surveys of external galaxies to achieve this.

[] Ulrich Kolb's home page. [] Astronomy Research at the OU.

Ulrich Kolb (U.C.Kolb@open.ac.uk)