Ejection of pulsars and binaries to the outskirts of globular clusters

Abstract

THREE-BODY interactions can eject stars, singly or in binaries, from the core of a globular cluster to its outskirts, whither dynamical friction may take more than 10^8 yr to return them. We show here that such a process can explain why the binary pulsar 2127 + 11C in the cluster Ml5 (and perhaps 1744–24A in Terzan 5) is now far from the cluster core. A suitable encounter could have given the pulsar enough velocity to eject it to its present position, and also replaced its original stellar companion with a neutron star. For ejection of PSR2127 +11C to be likely, the core of M15 must be composed of heavy degenerate stars at a density greater than 10^7pc^(-3), maintained for ≳10^8yr; this is consistent with previous dynamical estimates. A natural combination of factors enables us to see 2127 + 11C: a binary of longer period could not have received a large enough impulse to escape the core, whereas one of shorter period would have been ejected from the cluster or would have collapsed because of orbital decay by gravitational radiation. Pulsars and pulsar binaries ejected from clusters will contribute to the birth rate of recycled pulsars in the inner Galaxy.

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