Detection Rates for Close Binaries via Microlensing

Microlensing is one of the most promising methods of reconstructing thestellar mass function down to masses even below the hydrogen-burning limit. Thefundamental limit to this technique is the presence of unresolved binaries,which can in principle significantly alter the inferred mass function. Here wequantify the fraction of binaries that can be detected using microlensing,considering specifically the mass ratio and separation of the binary. We findthat almost all binary systems with separations greater than $b \sim 0.4$ oftheir combined Einstein ring radius are detectable assuming a detectionthreshold of $3\%$. For two M dwarfs, this corresponds to a limiting separationof $\gsim 1 \au$. Since very few observed M dwarfs have companions atseparations $\lsim 1 \au$, we conclude that close binaries will probably notcorrupt the measurements of the mass function. We find that the detectabilitydepends only weakly on the mass ratio. For those events for which individualmasses can be determined, we find that binaries can be detected down to $b \sim0.2$.Comment: 19 pages including 6 figures. Uses phyyzx format. Send requests for higher quality figures to gaudi@payne.mps.ohio-state.ed