The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -- compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure

A model for the population of helium stars in the Galaxy I. Low-mass stars

By means of population synthesis we model the Galactic ensemble of helium stars. It is assumed that all helium stars are formed in binaries. Under this assumption, single helium stars are produced by the mergers of helium remnants of components of close binaries (mainly, by merging helium white dwarfs) and by disruption of binaries with helium components in supernovae explosions. The estimate of the total birthrate of helium stars in the Galaxy is 0.043 yr$^{-1}$, their total number is estimated as $4 \times 10^6$. The rate of binarity in the total sample is 76%. We construct a subsample of low-mass (M_{\rm He} \lesssim 2 \ms) helium stars limited by observational selection effects: stellar magnitude ($V_{\rm He} \leq 16$), ratio of stellar magnitudes of components in binaries ($V_{\rm He}\leq V_{\rm comp}$), lower limit of the semiamplitude of radial velocity that is necessary for discovery of binarity ($K_{min}=30$ km/s). The parameters of this ``observable'' sample are in satisfactory agreement with the parameters of the observed ensemble of sdB stars. In particular, in the selection-limited sample binarity rate is 58%. We analyze the relations between orbital periods and masses of helium stars and their companions in systems with different combinations of components. We expect that overwhelming majority ($\sim 90$%) of unobserved components in binary sdB stars are white dwarfs, predominantly, carbon-oxygen ones.Comment: 16 pages, 6 figures, submitted to Astronomy Reports, fig. 6 corrected, conclusions unchange