Is the Large Magellanic Cloud a Large Microlensing Cloud?

An expression is provided for the self-lensing optical depth of the thin LMCdisk surrounded by a shroud of stars at larger scale heights. The formula iswritten in terms of the vertical velocity dispersion of the thin diskpopulation. If tidal forcing causes 1-5 % of the disk mass to have a heightlarger than 6 kpc and 10-15 % to have a height above 3 kpc, then theself-lensing optical depth of the LMC is $0.7 - 1.9 \times 10^{-7}$, which iswithin the observational uncertainties. The shroud may be composed of brightstars provided they are not in stellar hydrodynamical equilibrium.Alternatively, the shroud may be built from low mass stars or compact objects,though then the self-lensing optical depths are overestimates of the trueoptical depth by a factor of roughly 3. The distributions of timescales of theevents and their spatial variation across the face of the LMC disk offerpossibilities of identifying the dominant lens population. In propitiouscircumstances, an experiment lifetime of less than 5 years is sufficient todecide between the competing claims of Milky Way halos and LMC lenses. However,LMC disks can sometimes mimic the microlensing properties of Galactic halos formany years and then decades of survey work are needed. In this caseobservations of parallax or binary caustic events offer the best hope forcurrent experiments to deduce the lens population. The difficult models todistinguish are Milky Way halos in which the lens fraction is low (< 10 %) andfattened LMC disks composed of lenses with a typical mass of low luminositystars or greater. A next-generation wide-area microlensing survey, such as theproposed ``SuperMACHO'' experiment, will be able to distinguish even thesedifficult models with just a year or two of data.Comment: 25 pages, 4 figures, The Astrophysical Journal (in press