Red Clump Stars as a Tracer of Microlensing Optical Depth

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Abstract
Zaritsky & Lin have recently suggested that the color-magnitude diagram of the Large Magellanic Cloud (LMC) contains evidence of foreground red clump stars. They interpret this as evidence of tidal debris or a dwarf galaxy at a distance of ~35 kpc that may be responsible for the large gravitational microlensing optical depth observed by the MACHO collaboration. I derive a relationship between the microlensing optical depth of such a foreground population and the observed density of foreground horizontal-branch stars such as the red clump population postulated by Zaritsky & Lin. Recent observational determinations for Population I and Population II stellar mass functions are used to show that the surface density of the foreground red clump stars claimed by Zaritsky & Lin implies a microlensing optical depth in the range τfg = (0.8–3.6) × 10−8, which is only 3%-13% of τLMC as determined by the MACHO collaboration. If the foreground population has a similar star formation history to the LMC, then the implied τfg is only 3%-4% of τLMC.
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