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[28.01] Follow-up Spitzer Space Telescope Observations of Five LMC Microlensing Events

N. Kallivayalil (Harvard University), B.M. Patten, C. Alcock (CfA), H.T. Nguyen, M.W. Werner (Jet Propulsion Laboratory, California Institute of Technology)

Over the past decade several teams have detected microlensing events towards the Large Magellanic Cloud with the rate of these events exceeding what was expected from previously known populations of stars. The interpretation of this excess remains controversial but one potential explanation 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 infrared, could be used effectively to place bounds on this hypothesis. In this regard we have carried out IRAC and MIPS photometry for five LMC microlensing events.

An analysis of these data showed that for event MACHO-LMC-5 the lens is most likely a disk M5 dwarf with an inferred mass of ~0.2 M_sun from its IRAC fluxes and colors (Nguyen et al. 2004). While IRAC cannot resolve the source and lens, a cool stellar lens in the Galactic disk could manifest itself as an infrared excess in the combined source+lens photometry. Indeed, MACHO-LMC-5 was detected in all 4 IRAC bands and the photometry clearly showed the red excess expected from the combination of the source and lens stars.

Using this event as a model, we present our analysis of the other four LMC microlensing events observed in this program. For all four events, we do not detect any infrared excess associated with a cool lens with either IRAC or MIPS 24-micron observations. We present upper limits for these events in the 5 bands observed and discuss the region of phase-space in terms of distance and magnitude that we can probe with Spitzer and hence what the results mean for the microlensing population.

This work is based on observations made with the Spitzer Space Telescope, which is operated by JPL under NASA contract 1407.

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.