The Companion of M4a: A Planet or a Star?

Abstract

The $\ddot P$ (Backer 1992, Backer et al. 1993, Thorsett et al. 1993) observed for PSR~B1620-26 in the globular cluster M4 is most likely due to external gravitational jerk from a bound companion. Solving for the $\ddot P$ observed, constrained by the observed $\dot P$, two natural solutions are a jovian mass in a $\gtorder 10 \ AU, e=0.3-0.5$ orbit, or a solar mass companion in a $\sim 50 \ AU$ highly eccentric orbit (Michel 1994, Rasio 1994). If the companion is solar mass, the system must be young, and its location outside the cluster core is somewhat of a mystery. If the companion is planetary, it was either exchanged with the secondary, or angular momentum transport in excretion disks and planet formation is extraordinarily efficient. The implications for planet formation around stellar systems are potentially profound.