The macho project Hubble Space Telescope follow-up: Preliminary results onthe location of the large Magellanic Cloud microlensing source stars

We attempt to determine whether the MACHO microlensing source stars are dra wn from the average population of the LMC or from a population behind the L MC by examining the Hubble Space Telescope (HST) color-magnitude diagram (C MD) of microlensing source stars. We present WFPC2 HST photometry of eight MACHO microlensing source stars and the surrounding fields in the LMC. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and the n transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that present ed by Zhao, Graff, & Guhathakurta. In this model, the source stars have an additional reddening of [E(B - V)] = 0.13 mag and a slightly larger distanc e modulus, [Delta mu] similar to 0.3 mag, than the average LMC population. We also investigate a series of source star models, varying the relative fr action of source stars drawn from the average and background populations an d the displacement of the background population from the LMC. Because of th e small number of analyzed events, the distribution of probabilities of dif ferent models is rather flat. A shallow maximum occurs at a fraction s(LMC) similar to 0.8 of the source stars in the LMC. This is consistent with the interpretation that a significant fraction of observed microlensing events are due to lenses in the Milky Way halo, but does not definitively exclude other models.