The MACHO ProjectHubble Space TelescopeFollow‐Up: Preliminary Results on the Location of the Large Magellanic Cloud Microlensing Source Stars

We attempt to determine whether the MACHO microlensing source stars are drawnfrom the average population of the LMC or from a population behind the LMC byexamining the HST color-magnitude diagram (CMD) of microlensing source stars.We present WFPC2 HST photometry of eight MACHO microlensing source stars andthe surrounding fields in the LMC. The microlensing source stars are identifiedby deriving accurate centroids in the ground-based MACHO images usingdifference image analysis (DIA) and then transforming the DIA coordinates tothe HST frame. We consider in detail a model for the background population ofsource stars based on that presented by Zhao, Graff & Guhathakurta. In thismodel, the source stars have an additional reddening = 0.13 mag and aslightly larger distance modulus ~ 0.3 mag than the average LMCpopulation. We also investigate a series of source star models, varying therelative fraction of source stars drawn from the average and backgroundpopulations and the displacement of the background population from the LMC. Dueto the small number of analyzed events the distribution of probabilities ofdifferent models is rather flat. A shallow maximum occurs at a fraction s_LMC ~0.8 of the source stars in the LMC. This is consistent with the interpretationthat a significant fraction of observed microlensing events are due to lensesin the Milky Way halo, but does not definitively exclude other models.Comment: revised version, results slightly changed, accepted by Ap