Neutron star/red giant encounters in globular clusters

Abstract

It has been suggested that the tidal capture of a red giant by a neutron star in globular clusters may result in bound systems¹, the subsequent evolution of which may lead to a variety of interesting objects, including millisecond pulsars^2,3. But, the binding energy of the envelope of a giant is much less than that of a main-sequence star⁴, so the ratio x_crit = R_crit/R*(where R_crit is the maximum distance of closest approach between two stars for which the tidal energy is sufficient to bind the system, and R* is the radius of the star on which tides are being raised) will be less for giants than for main-sequence stars. Here I present a simple expression for the amount by which x_crit is diminished as a star evolves, and conclude that tidal capture of giants by neutron stars resulting in binary systems is unlikely in globular clusters. However, collisions between neutron stars and red giants^3,5,6, or an alternative process involving tidal capture of a main-sequence star into an initially detached binary system⁷, may result either in rapidly rotating neutron stars or in white dwarf/neutron star binaries.