THE ORBITAL ECCENTRICITIES OF BINARY MILLISECOND PULSARS IN GLOBULAR CLUSTERS

Abstract

Low-mass binary millisecond pulsars (LMBPs) are born with very small orbital eccentricities, typically of order $e_i\sim10^{-6}$--$10^{-3}$. In globular clusters, however, higher eccentricities $e_f\gg e_i$ can be induced by dynamical interactions with passing stars. Here we show that the cross section for this process is much larger than previously estimated. This is because, even for initially circular binaries, the induced eccentricity $e_f$ for an encounter with pericenter separation $r_{\rm p}$ beyond a few times the binary semi-major axis $a$ declines only as a power-law, $e_f\propto(r_{\rm p}/a)^ {-5/2}$, and {\it not\/} as an exponential. We find that {\it all\/} currently known \lmbps in clusters were probably affected by interactions, with their current eccentricities typically an order of magnitude or more greater than at birth.