On the formation of red giant binaries by tidal capture

The nonradial oscillation spectra of a 0.8 M_⊙ 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 M_⊙ neutron star with a relative velocity at infinity of 10 km s^-1, the critical impact parameter, x_crit≡ R_crit/R*, decreases as the star evolves, ranging from 2.42 at the base of the subgiant branch to ∼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 3 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 neutron stars.