K giants as astrometric reference stars for the Space Interferometry Mission

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Abstract
Wide angle astrometry with the Space Interferometry Mission needs a set of several thousand grid stars distributed uniformly over the sky. The requirements for candidate grid stars are quite stringent: the photocenters of these stars have to be astrometrically stable to within a few microarcseconds, which makes binary stars unacceptable as grid stars. We search the most precise and comprehensive astrometric catalogs available today--the Hipparcos, Tycho-1 and Tycho-2 Catalogues--for possible grid stars, and discuss the properties of samples of K giants derived from these catalogs. Furthermore, we present results of a precise radial velocity study of a small proxy sample of Hipparcos K giants. We demonstrate that it is possible to find K giants with radial velocity variations smaller than a few tens of meters per second on timescales of several months. It is thus possible to detect stellar companions in samples of candidate grid stars by means of a radial velocity survey. We discuss the results of Monte Carlo simulations that address the consequences of the measurement accuracy of a radial velocity survey on the portion of undetected binary systems.

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