ASSESSING THE ENVIRONMENTAL-IMPACT ON PSR B1620-26 IN M4

The P of the binary millisecond pulsar PSR B1620-26 in the Galactic gl obular cluster M4 (Backer 1992; Backer, Sallmen, & Foster 1993; Thorse tt, Arzoumanian, & Taylor 1993), indicates the pulsar is a member of a hierarchical triple. The tertiary may have a mass, of from similar to 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 constrai n the mass, m(2), semimajor axis, a(2), eccentricity, e(2), and angle between the line of sight and the semimajor axis, omega(2), of the ter tiary. We consider the expected values of some of the observable varia bles for different values of m(2), a(2), and e(2), and we show that a nonzero e(2) permits a surprisingly large range of values for m(2), a( 2). In particular, the apparent mean motion provides a poor measure of the tertiary orbital period when e(2) similar to 0.3. We consider per turbations of the inner binary orbital parameters, in particular, the inner binary orbital period, P-1. Measurements of higher time derivati ves of the spin period, and time derivatives of the orbital elements o f the inner binary, will soon provide very strong constraints on the o rbital parameters of the system. We also discuss scenarios for formati on and subsequent evolution of planetary and stellar triples in M4, an d the implications for PSR B1620-26. If the tertiary is substellar, th e 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 f ield star. If the tertiary is of stellar mass, the system is likely to be younger than inferred from its characteristic age and to have unde rgone multiple encounters with field stars. The confirmation of PSR B1 620-26 as a hierarchical triple pulsar would provide fascinating insig ht into pulsar formation and stellar dynamics in grobular clusters. A planetary mass tertiary would offer strong evidence for planet formati on being common in solar-type stars, even those of low metallicity.