The MACHO Project: Limits on Planetary Mass Dark Matter in the Galactic Halo from Gravitational Microlensing

The MACHO project has been monitoring about 10 million stars in the Large Magellanic Cloud (LMC) in the search for gravitational microlensing events caused by massive compact halo objects (MACHOs) in the halo of the Milky Way. In our standard analysis, we have searched this data set for well-sampled, long-duration microlensing light curves, detected several microlensing events consistent with MACHOs in the 0.1 M☉ ≲ m ≲ 1.0 M☉ mass range, and set limits on the abundance of objects with masses 10−5 M☉ ≲ m ≲ 10−1 M☉. In this paper, we present a different type of analysis involving the search for very short timescale brightenings of stars, which is used to set strong limits on the abundance of lower mass MACHOs. Our analysis of the first 2 years of data toward the LMC indicates that MACHOs with masses in the range 2.5 × 10–7 M☉ < m < 5.2 × 10−4 M☉ cannot make up the entire mass of a standard spherical dark halo. Combining these results with those from the standard analysis, we find that the halo dark matter cannot be comprised of objects with masses 2.5 × 10−7 M☉ < m < 8.1 × 10−2 M☉.