Nearby Microlensing Events: Identification of the Candidates for theSpace Interferometry Mission

The Space Interferometry Mission (SIM) is the instrument of choice when itcomes to observing astrometric microlensing events where nearby, usuallyhigh-proper-motion stars (``lenses''), pass in front of more distant stars(``sources''). Each such encounter produces a deflection in the source'sapparent position that when observed by SIM can lead to a precise massdetermination of the nearby lens star. We search for lens-source encountersduring the 2005-2015 period using Hipparcos, ACT and NLTT to select lenses, andUSNO-A2.0 to search for the corresponding sources, and rank these by the SIMtime required for a 1% mass measurement. For Hipparcos and ACT lenses, the lens distance and lens-source impactparameter are precisely determined so the events are well characterized. Wepresent 32 candidates beginning with a 61 Cyg A event in 2012 that requiresonly a few minutes of SIM time. Proxima Centauri and Barnard's star eachgenerate several events. For NLTT lenses, the distance is known only to afactor of 3, and the impact parameter only to 1''. Together, these produceuncertainties of a factor ~10 in the amount of SIM time required. We present alist of 146 NLTT candidates and show how single-epoch CCD photometry of thecandidates could reduce the uncertainty in SIM time to a factor of ~1.5.Comment: ApJ accepted, 31 pages (inc. 5 tables), 5 figures. t SIM refine