Interpreting the M22 Spike Events

Abstract

Recently Sahu et al., using the Hubble Space Telescope to monitor stars in the direction of the old globular cluster M22, detected six events in which otherwise constant stars brightened by ~50% during a time of <1 day. They tentatively interpret these unresolved events as due to microlensing of background bulge stars by free-floating planets in M22. I show that if these spike events are due to microlensing, the lensing objects are unlikely to be associated with M22, and unlikely to be part of a smoothly distributed Galactic population. Thus either there happens to be a massive, dark cluster of planets along our line-of-sight to M22, or the spike events are not due to microlensing. The lensing planets cannot be bound to stars in the core of M22: if they were closer than 8 AU, the lensing influence of the parent star would have been detectable. Moreover, in the core of M22, all planets with separations > 1 AU would have been ionized by random stellar encounters. Most unbound planets would have escaped the core via evaporation which preferentially affects such low-mass objects. Bound or free-floating planets can exist in the outer halo of M22; however, for reasonable assumptions, the maximum optical depth to such a population falls short of the observed optical depth, tau ~ 3x10^{-6}, by a factor of 5-10. Therefore, if real, these events represent the detection of a significant free-floating Galactic planet population. The optical depth to these planets is comparable to and mutually exclusive from the optical depth to resolved events measured by microlensing survey collaborations toward the bulge, and thus implies a similar additional mass of lensing objects. Such a population is difficult to reconcile with both theory and observations.Comment: Minor changes. 12 pages, 4 figures, 2 tables. Accepted to ApJ. To appear in Feb 10, 2002 issue (v566