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E. J. McGrath (Vassar College), K. C. Sahu (STScI)
Using Hubble Space Telescope (HST) archive data, we have compared the fields of previously detected microlensing events towards the Large Magellanic Cloud (LMC) with the ground based observations of the MACHO Project. At full resolution (0.045"/pix), the HST images often reveal several stars in what appears to be a single star in the ground based image. Although blending has been recognized to be an important factor, previous studies have not fully taken this effect into account when calculating the microlensing optical depth. A careful determination of the microlensing optical depth is important for statistical determination of the location of the lenses since, for the majority of microlensing events, it is impossible to calculate the distance to the lens directly. The microlensing optical depth thus determined can be compared with the values expected from known stellar populations in the LMC and assumed dark matter populations within the Milky Way halo in order to determine where the lensing objects reside. When this blending effect is taken into account, we find that the value for the measured microlensing optical depth decreases, since the number of monitored stars is larger than what the ground based observations would imply. The resultant optical depth tends to support the idea that the observed microlensing events may be caused by stars within the LMC rather than by MACHOs in the Galactic halo. Some results will also be presented on the simulations of the light curves taking the blending effects into account.
We acknowledge support from the STScI Summer Student Program.
The author(s) of this abstract have provided an email address for comments about the abstract: elmcgrath@vassar.edu