The LMC Microlensing Events Originate in the Warped and Flaring Milky Way Disk
Abstract
The MACHO and EROS Collaborations, monitoring millions of stars in the Large Magellanic Cloud (LMC), have seen at least fourteen instances of microlensing. The simplest interpretation is that about one third of the halo of our own Milky Way galaxy exists in the form of objects of around 0.5 solar mass. There are problems with this interpretation. A normal stellar population of 0.5 solar mass stars should be visible. The remaining candidate for the lenses is a population of white dwarfs. But, the precursor population must have enriched the interstellar medium with metals, in conflict with current population II abundance ratios. Moreover, the mass budget is very high, because the efficiency of making white dwarfs is only around ten per cent. Here, we propose a more conventional, but at the moment more speculative, explanation. Some of the lenses are stars in the disk of the Milky Way a few kiloparsecs from us. They lie along the line of sight to the LMC because of warping and flaring of the Galactic disk. Microlensing towards the LMC is telling us more about the structure and stellar populations of the outer Milky Way disk than the composition of the dark halo.Keywords
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