Microlensing planets in M 22: Free-floating or bound?

Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain

Corresponding author: R. de la Fuente Marcos, rfuente@ucmail.ucm.es

Received: 9 July 2001
Accepted: 31 August 2001

Abstract

We use detailed numerical simulations and theoretical estimates to show that, if confirmed, the unusually brief microlensing events observed by Sahu et al. ([CITE]) in the field of the globular cluster M 22 might be explained as a result of microlensing by a population of clustered MACHOs, a dark cluster or RAMBO, not associated with the globular cluster. If real, this dark cluster would be located between M 22 and the Galactic bulge and could include at least 10⁶ substellar members with a typical size of 1-3 pc. Bound planets in wide or/and eccentric orbits are also able to reproduce the observed microlensing behaviour, but only if multiplanet systems (including large Kuiper-belt-like objects) are abundant, although, our calculations argue against the latter scenario as the ionization rate in M 22 is very high. Dynamically ejected or lone planets are, in principle, incompatible with the observational findings as they either escape their parent cluster in a relatively short time-scale after ejection or segregate toward the outskirts of the cluster. We discuss additional implications of the dark cluster scenario, including the existence of a population of RAMBOs toward the Galactic bulge.