LMC stars vs . the location of the lenses

We propose an observational test that can break the degeneracy of two main classes of microlensing models to the Magellanic Clouds: (a) the lenses are located in the Galactic halo, and (b) the lenses are located in the LMC disk. The source stars in the latter (self-lensing) models tend to be at the far side or behind the LMC disk, thus experience more reddening and extinction by dust in the LMC disk than ordinary stars in a nearby line of sight. Clearly such bias would not occur in the macho halo lensing models. We show that this reddening effect is at a level readily observable for the present 30 or so microlensing alerts fields, either with multi-band photometry from a good seeing site, or more definitively with ultra-violet (UV) spectroscopy with STIS or COS of Hubble Space Telescope (HST). Stars behind the LMC dust layer should stand out as UV-faint objects (by more than 1 mag than average stars in the LMC). HST can also resolve numerous faint neighbouring stars within a few arcsecs of a lensed source, hence remove blending in these crowded regions and build a reddening map to control the patchiness of dust. Subject headings: dust, extinction – Magellanic Clouds — Galaxy: structure