THE FORMATION RATE OF LOW-MASS X-RAY BINARIES IN GLOBULAR-CLUSTERS

We estimate the lifetime of low-mass X-ray binaries (LMXBs), based on a theoretical formation rate in globular clusters together with the observed current abundance. We argue that the formation rate may be more efficient than previously estimated, by an order of magnitude or more. Two arguments lead in this direction: (1) using a weighted average of globular cluster escape velocities, we find a higher fraction of neutron stars, about 0.3, being retained in globular clusters; (2) using recent, higher estimates of the number of primordial binaries in globular clusters, we find an efficient capture of a large fraction of neutron stars through exchange reactions. We conclude that LMXBs may have a typical lifetime of 10(8) yr or less. Such a short lifetime could explain the large number of millisecond radio pulsars, both in the Galaxy as well as in globular clusters, and would not contradict the standard scenario of LMXBs producing millisecond pulsars. In addition, our arguments naturally lead to relatively long orbital periods and young ages for millisecond pulsars in globular clusters, both of which are in agreement with observations.