Identifying Microlensing Source Stars with HST and Difference Imaging

Details Identifying Microlensing Source Stars with HST and Difference Imaging Identifying Microlensing Source Stars with HST and Difference Imaging

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.4803n&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #48.03; Bulletin of the American Astronomical Society, Vol. 31, p.1444

Astronomy and Astrophysics

Astronomy

Scientific paper

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.

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