Nearby microlensing events:: Identification of the candidates for the Space Interferometry Mission

The Space Interferometry Mission (SIM) is the instrument of choice when it comes to observing astrometric microlensing events where nearby, usually high proper motion, stars ("lenses") pass in front Of more distant stars ("sources"). Each such encounter produces a deflection in the source's apparent position that, when observed by STM, can lead to a precise mass determination of the nearby lens star. We search for lens-source encounters during the 2005-2015 period using Hipparcos, ACT, and NLTT to select lenses, and USNO-A2.0 to search for the corresponding sources, and rank these by the SIM time required for a 1% mass measurement. For Hipparcos and ACT lenses, the lens distance and lens-source impact parameter are precisely determined so that the events are well characterized. We present 32 candidates beginning with a 61 Cyg A event in 2012 that requires only a few minutes of SIM time. Proxima Centauri and Barnard's star each generate several events. For NLTT lenses, the distance is known only to a factor of 3, and the impact parameter only to 1". Together, these produce uncertainties of a factor similar to 10 in the amount of SIM time required. We present a list of 146 NLTT candidates and show how single-epoch CCD photometry of the candidates could reduce the uncertainty in SIM time to a factor of similar to 1.5.