RETENTION FRACTIONS FOR GLOBULAR CLUSTER NEUTRON STARS

ABSTRACT Fokker-Planck models are used to give estimates for the retention fractions for newly-born neutron stars in globular clusters as a function of kick velocity. These can beused to calculate the present day numbers of neutron stars in globular clusters andin addressing questions such as the origin of millisecond pulsars. As an example, thePopulation I kick velocity distribution of Lyne & Lorimer (1994) is used to estimatethe retained fractions of neutron stars originatingas single stars and in binary systems.For plausible initial conditions fewer than 4% of single neutron stars are retained. Theretention fractions from binary systems can be 2 to 5 times higher. The dominantsource of retained neutron stars is found to be through binary systems which remainbound after the first supernova, ie. high-mass X-ray binaries. The retained fraction de-creases with an increasing number of progenitors, but the retention fraction decreasesmore slowly than the number of progenitors increases. On balance, more progenitorsgive more neutron stars in the cluster.Key words: globular clusters: general – stars: neutronTo appear in MNRAS.