Analytic Study of Grid Star and Reference Star Selection for the Space Interferometry Mission

Grid stars and reference stars provide the fundamental global and local astrometric reference frames for observations by the Space Interferometry Mission. They must therefore be astrometrically stable at the ~1 μas level. I present simple formulae in closed form to estimate the contamination of these frames by motions due to stellar companions that go undetected in a radial-velocity (RV) survey of specified precision. The contamination rate depends almost entirely on the binary-period distribution function and not on the details of the mass or eccentricity distributions. Screening by a modest RV survey (σ_RV = 60 m s^-1) can reduce the fraction of grid stars with detectable unmodeled accelerations to <