Birth of millisecond pulsars in globular clusters

Abstract

The recent report that two millisecond pulsars were found in the globular cluster 47 Tuc¹, together with the previous discoveries in the clusters M28², M4³ and M15⁴, indicates that formation of millisecond pulsars in globular clusters is greatly enhanced over the field, where a fourth example has recently been found⁵. This excess appears to be even larger than that for low-mass X-ray binaries (LMXBs), which show a factor of 100 excess (per unit mass) in globular clusters as opposed to the Galaxy^6,7. If the birthrate of millisecond pulsars is indeed greater than that of LMXBs in clusters, then, contrary to the common assumption⁸, LMXBs cannot be the sole precursors of millisecond pulsars. Here we suggest instead that accretion-induced collapse of white dwarfs in binaries⁹ can form millisecond pulsars directly, without requiring a precursor LMXB stage¹⁰. Ablation of the pre-collapse binary companion by the millisecond pulsar's radiation field¹¹, a process invoked to explain some of the characteristics of the recently discovered eclipsing millisecond pulsar^5,12,13, can then yield isolated neutron stars without requiring an additional stellar encounter.