Using Optical and Near‐Infrared Photometry to Test MACHO Lens Candidates

Abstract

We obtained new VLT/ISAAC H-band observations for five MACHO LMC source stars and adjacent LMC field regions. After combining our near-IR photometry with Hubble Space Telescope/PC BVRI optical photometry, we compared the MACHO objects to the adjacent field stars in a variety of color-magnitude and color-color diagrams. These diagnostic diagrams were chosen to be sensitive to our hypothesis that at least some of the MACHO lenses were foreground Galactic disk or thick-disk M dwarfs. For the five lensed objects we studied, our hypothesis could be ruled out for main-sequence lens masses 0.1 M? for distances out to 4 kpc. On the other hand, the fact that LMC MACHO 5, an object not in our study, has been recently found to have just such a foreground lens highlights that the remainder of the LMC MACHO objects should be searched for the signature of their lenses using our photometric technique or via near-IR spectroscopy. We also constructed diagnostic color-color diagrams sensitive to determining reddening for the individual MACHO source stars and found that these five objects did not show evidence for significant additional reddening. At least these five MACHO objects are thus also inconsistent with the LMC self-lensing hypothesis.