Selection of Nearby Microlensing Candidates for Observation by SIM

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I investigate the prospects for using the Space Interferometry Mission (SIM) to measure the masses of nearby stars from their astrometric deflection of more distant sources, as originally suggested by Paczynski and by Miralda-Escude. I derive an analytic expression for the total observing time Tₜot required to measure the masses of a fixed number of stars to a given precision. I find that Tₜot rₘax⁻², where rₘax is the maximum radius to which candidates are searched, or Tₜot μₘin², where μₘin is the minimum proper motion to which candidates are searched. I show that Tₜot can be reduced by a factor 4 if source availability is extended from Vₛ=17 to Vₛ=19. Increasing rₘax and Vₛ and decreasing μₘin all require a significantly more agressive approach to finding candidates. A search for candidates can begin by making use of the Luyton proper motion catalog together with the USNO-A2.0 all-sky astrometric catalog. However, a thorough search would require the all-sky USNO-B proper-motion catalog which is not yet available. The follow-up observations necessary to prepare for the mission will become more difficult the longer they are delayed because the candidate pairs are typically already within 1" and are getting closer.