THE NEUTRON-STAR HELIUM WHITE-DWARF POPULATION IN THE GALACTIC DISK

The population of low-mass binary pulsars in the Galactic disk has gro wn considerably in the past few years. By ''low mass,'' we mean that t he companion to the neutron star is, or is very likely to be, a helium white dwarf. The distribution of these objects in the P-(P) over dot plane, where P is the rotation period, suggests both that their initia l periods after spin-up are near 3 ms and that the age of the populati on is near 5 Gyr. This initial spin-up is similar to the inferred rota tion rates in low-mass X-ray binaries (LMXBs) with dual-line quasi-per iodic oscillations at kilohertz frequencies. Furthermore, the age of 5 Gyr is consistent with models of the LMXB population as evolving from stars with masses just above 1 M-., although significant uncertaintie s remain in observationally derived birthrates of the radio and X-ray binaries. The observed distribution of minimum masses in the low-mass binary pulsar population requires a distribution of actual masses and is not the result of a random distribution of inclination angles. This distribution further leads to the conclusion that these millisecond p ulsar systems do not have random inclination angles. A preference for orthogonal magnetic and spin axes is a possible configuration.