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., similar to 10(8) yr above 10(36) ergs s(-1) for a 1.4 M-. neutron star accreting from a degenerate helium companion with an initial mass of similar to 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 similar to 10(6) - 10(10) yr, usually shorter and often much shorter than the cluster age.