Assessing the environmental impact on PSR B1620-26 in M4

The P̈ of the binary millisecond pulsar PSR B1620-26 in the Galactic globular cluster M4 (Backer 1992; Backer, Sallmen, & Foster 1993; Thorsett, Arzoumanian, & Taylor 1993), indicates the pulsar is a member of a hierarchical triple. The tertiary may have a mass, of from ∼10^-3 M_⊙ to 1 M_⊙, and orbits the inner binary with a semimajor axis of between 10 and 50 AU. The observed spin period derivatives constrain the mass, m₂, semimajor axis, a₂, eccentricity, e₂, and angle between the line of sight and the semimajor axis, ω₂, of the tertiary. We consider the expected values of some of the observable variables for different values of m₂, a₂, and e₂, and we show that a nonzero e₂ permits a surprisingly large range of values for m₂, a₂. In particular, the apparent mean motion provides a poor measure of the tertiary orbital period when e₂ ∼ 0.3. We consider perturbations of the inner binary orbital parameters, in particular, the inner binary orbital period, P₁. Measurements of higher time derivatives of the spin period, and time derivatives of the orbital elements of the inner binary, will soon provide very strong constraints on the orbital parameters of the system. We also discuss scenarios for formation and subsequent evolution of planetary and stellar triples in M4, and the implications for PSR B1620-26. If the tertiary is substellar, the system must have spent a large fraction of its lifetime outside the core of M4 and may have survived one or more close encounters with a field star. If the tertiary is of stellar mass, the system is likely to be younger than inferred from its characteristic age and to have undergone multiple encounters with field stars. The confirmation of PSR B1620-26 as a hierarchical triple pulsar would provide fascinating insight into pulsar formation and stellar dynamics in globular clusters. A planetary mass tertiary would offer strong evidence for planet formation being common in solar-type stars, even those of low metallicity.