SIMBAD references

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 SIM, 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 ∼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 ∼1.5.