AAS 195th Meeting, January 2000
Session 48. Lensing
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

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[48.03] Identifying Microlensing Source Stars with HST and Difference Imaging

C.A. Nelson, C. Alcock, K. Cook, M. Geha, S.L. Marshall, P. Popowski (IGPP), D.L. Welch (McMaster U.), A.C. Becker, M. Pratt, C.W. Stubbs, A.B. Tomaney (U. Washington), T.S. Axelrod, A.J. Drake, K.C. Freeman, B.A. Peterson (MSSSO), K. Griest, T. Vandehei (UCSD), R.A. Allsman (ANUSF), D.R. Alves (STSCI), D.P. Bennett (U. Notre Dame), M.J. Lehner (U. Sheffield), D. Minniti (P. Universidad Catolica), W. Sutherland (Oxford), P.J. Quinn (ESO), MACHO Collaboration

To date, the MACHO project has identified approximately 20 microlensing events towards the Large Magellanic Cloud (LMC), each event appearing as a temporary brightening in MACHO photometry of LMC stars. Due to crowding in our ground-based photometry, these ``stars'' are usually not single stars at all, but rather blended objects made up of several fainter stars. By combining the technique of difference imaging with high resolution HST/WFPC2 images we can identify which of the many resolved HST stars constituting one MACHO object was the microlensed star. Difference imaging is used to locate the centroid of the lensed flux in the ground-based MACHO image to an accuracy of around 0.1 to 0.2 arcseconds. We then derive coordinate transformations between the MACHO and HST frames and, in most cases, can unambiguously identify the lensed star. This information allows us to verify ground-based estimates of parameters important to the calculation of the optical depth such as the unlensed, baseline flux, which is critical in determining the Einstein Ring crossing time. It also allows us to verify that lensed stars do not cluster to any particular area of the color-magnitude diagram. Finally, the unblended colors allow us to comment on LMC self-lensing by determining if the lensed stars are more reddened than their neighbours, and therefore more likely to lie on the far side of the LMC disk.

The author(s) of this abstract have provided an email address for comments about the abstract: cnelson@igpp.llnl.gov

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