Evolution of stellar binaries formed by tidal capture

Abstract

Two-body tidal capture, as proposed by Fabian et al. (1975), is the favored mechanism for the formation of X-ray binaries in globular clusters. The tidal capture formation and subsequent evolution of a system consisting of a neutron star and a low mass main sequence star is considered, yielding the amount of tidal energy deposited during the first and later close passages, and the radial distribution of this energy. Going further, the effects of the viscous dissipation of the tidal energy on the structure of the low-mass star and on the binary system are examined. The tidal energy is thermalized on a timescale of 10⁴ yr. The consequent high tidal luminosity causes the star to expand and overflow its Roche lobe, resulting in the formation of a common envelope. This makes the stellar core and the neutron star spiral towards each other because of the frictional drag. The state reached by the system after the dissipation of the tidal energy depends on the relative values of the various timescales relevant to the system. Depending on these values the system may evolve into any of the following configurations: an X-ray binary, a detached binary, a neutron star surrounded by a massive accretion disk and a cloud of matter, or a Thorne-Zytkow object.