Halo Lensing or Lmc Self Lensing? Insights from the Hst Cmd of Macho Microlensing Source Stars

The nature of Large Magellanic Cloud (LMC) microlensing is determined by the location of the microlensing source stars. If the source stars are in the LMC then LMC microlensing is predominantly halo-lensing, if the source stars are located behind the LMC, then LMC microlensing is dominated by self-lensing. We attempt to distinguish between source stars drawn from the average population of the LMC and source stars drawn from a population behind the LMC by examining the HST color-magnitude diagram (CMD) 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 then transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that presented by Zhao, Graff & Guhathakurta. In this model, the source stars have an additional reddening < E(B − V ) >= 0.13 mag and a slightly larger distance modulus < ∆μ >∼ 0.3 mag than the average LMC population. We also investigate a series of source star models, varying the relative fraction of source stars drawn from the average and background populations and the displacement of the background population from the LMC. The data suggest that all of the MACHO microlensing source stars are not drawn from a background population. 1 Lawrence Livermore National Laboratory, Livermore, CA 94550 Email: alcock, kcook, adrake, cnelson, popowski, [email protected]