On the formation of red giant binaries by tidal capture

Abstract

The nonradial oscillation spectra of a 0.8 solar mass star in various evolutionary stages along the red giant branch have been studied in the context of tidal capture binary formation in globular clusters. For capture of a 1.4 solar mass neutron star with a relative velocity at infinity of 10 km/s, the critical impact parameter, X(crit), decreases as the star evolves, ranging from 2.42 at the base of the subgiant branch to about 1.45 at the onset of helium burning. For the capture of lighter stars by the giant, or for larger relative velocities, X(crit) decreases. The reduction of X(crit) from the usually adopted value of three leads to a diminished frequency of binary systems containing a red giant companion and increases the likelihood of the formation of ultra-short period compact binary systems and mergers. The existence of the binary millisecond radio pulsar PSR 1620-26 in the globular cluster M4 indicates that disruption can be avoided in some cases and that the effects of tidally enhanced expansion are minimal for red giant-neutron star binary progenitor systems of this type. An evolutionary scenario for PSR 1620-26 is constructed, demonstrating the viability of the tidal capture scenario for red giants and neutronmore » stars. 34 refs.« less