Using Astrometry to Deblend Microlensing Events

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Abstract
We discuss the prospect of deblending microlensing events by observing astrometric shifts of the lensed stars. Since microlensing searches are generally performed in very crowded fields, it is expected that stars will be confusion limited rather than limited by photon statistics. By performing simulations of events in crowded fields, we find that if we assume a dark lens and that the lensed star obeys a power-law luminosity function, n(L) ∝ L-β, over half the simulated events show a measurable astrometric shift. Our simulations included 20,000 stars in a 256 × 256 Nyquist-sampled CCD frame. For β = 2, we found that 58% of the events were significantly blended (F*/Ftot ≤ 0.9), and of those, 73% had a large astrometric shift (≥0.5 pixels). Likewise, for β = 3, we found that 85% of the events were significantly blended and that 85% of those had large shifts. Moreover, since the shift may be used to determine the true position of a source star with respect to the observed point-spread function, a high-resolution follow-up survey may be used to identify the source star and determine the blending fraction directly.
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