AAS 204th Meeting, June 2004
Session 41 Spitzer Space Telescope
Topical Related Poster, Tuesday, June 1, 2004, 10:00am-7:00pm, Ballroom

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[41.21] Spitzer Space Telescope Observations of the Aftermath of Microlensing Event MACHO-LMC-5

N. Kallivayalil (University of Pennsylvania), H. T. Nguyen, M. W. Werner (Jet Propulsion Laboratory, California Institute of Technology), C. Alcock (University of Pennsylvania), B. M. Patten (Harvard-Smithsonian Center for Astrophysics), D. Stern (Jet Propulsion Laboratory, California Institute of Technology)

Over the past decade a number of teams have clearly detected microlensing towards the Large Magellanic Cloud with the rate of microlensing exceeding what is expected from previously known populations of stars. The interpretation of this excess remains controversial but one of the potential explanations is that the lenses belong to a previously undetected cool component of the disk of the Milky Way. Spitzer with its unprecedented sensitivity in the infra-red could be used very effectively to place bounds on this hypothesis. In this regard we have carried out photometry with the IRAC of the LMC star that was magnified during microlensing event MACHO-LMC-5. This event is unique in the annals of gravitational microlensing: it is the only event for which the lens itself has been observed (once with WFPC2 and twice with ACS/HRC both on the HST). The recent ACS/HRC data confirm that this event shows the phenomenon of ``jerk parallax'', which allowed the identification of the lens as an M dwarf at ~600 pc. The separation between source and lens at the epoch of the Spitzer observations was ~ 0.24'', and thus the two stars cannot be resloved in the Spitzer images, but a cool stellar lens in a Galactic disk population typically will show up as an infrared excess. In fact MACHO-LMC-5 is detected in all 4 IRAC bands and the photometry clearly shows the red excess expected from the combination of the source and lens star. We report from the colors that the lens is a late M dwarf. We also confirm from this study that we can use this event as a model for further analysis of other LMC events proving that Spitzer will be a very useful tool for studying MACHOs.

This work is based [in part] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through an award issued by JPL/Caltech.

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