Formation of ultracompact x-ray binaries in dense star clusters

Bright, ultracompact X-ray binaries observed in dense star clusters, such as Galactic globular clusters, must have formed relatively recently, since their lifetimes as persistent bright sources are short (e.g., ∼10 ⁸ yr above 10³⁶ ergs s^-1 for a 1.4 M⊙ neutron star accreting from a degenerate helium companion with an initial mass of ∼0.2 M⊙). Therefore, we can use the present conditions in a cluster core to study possible dynamical formation processes for these sources. Here we show that direct physical collisions between neutron stars and red giants can provide a sufficient formation rate to explain the observed numbers of bright sources. These collisions produce tight, eccentric neutron star-white dwarf binaries that decay to contact by gravitational radiation on timescales ∼10⁶-10¹⁰ yr, usually shorter and often much shorter than the cluster age.