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C.A. Nelson, K.H. Cook, P. Popowski, A.J. Drake, S.L. Marshall (IGPP/LLNL), K. Griest, T. Vandehei (UC San Diego), C. Alcock (UPenn), R.A. Allsman, T.S. Axelrod, K.C. Freeman, B.A. Peterson (ANU), D.R. Alves (STScI), A.C. Becker, C.W. Stubbs, A.B. Tomaney (Washington), D.P. Bennett (Notre Dame), M. Geha (UC Santa Cruz), M.J. Lehner (Sheffield), D. Minniti (Catolica), M.R. Pratt (CSR/MIT), P.J. Quinn (ESO), W. Sutherland (Oxford), D. Welch (McMaster), MACHO Collaboration
There are two principle geometrical arrangements which may explain Large Magellanic Cloud (LMC) microlensing: a) halo-lensing, in which the lensed object is part of the Milky Way galactic halo and b) self-lensing, in which the lensed object is part of the LMC. Self-lensing in turn may be broken into two categories: LMC-LMC self-lensing, in which both the source and the lens reside in the LMC and background self-lensing, in which the lens is a star in the LMC and the source star is drawn from some population behind the LMC. Models suggest the contribution of LMC-LMC self-lensing is small, so the nature of LMC microlensing may be estimated from the location of the microlensing source stars. If the source stars are in the LMC then microlensing is dominated by halo-lensing; conversely if the source stars are located behind the LMC then microlensing is dominated by self-lensing. Since background populations reside behind the LMC, we expect them to be both redder and fainter then the average population of the LMC. We attempt to determine if the MACHO source stars come from such a background population by comparing the HST color-magnitude diagram (CMD) of source stars to the CMD of the average population of the LMC and looking for the effects of extra reddening and extinction. 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. Preliminary results suggest that halo-lensing accounts for \gtrsim 40% of the observed microlensing results.
Support provided by NASA, DOE, NSF and NPSC.