A strategy for identifying the grid stars for the Space Interferometry Mission

We present a strategy to identify several thousand stars that are astrometr ically stable at the microarcsecond level for use in the Space Interferomet ry Mission (SIM) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into acc ount, we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star owing to their intrinsic brightness. We show that it is possible to i dentify suitable candidate grid K giants from existing astrometric catalogs . However, double stars have to be eliminated from these candidate grid sam ples, since they generally produce much larger astrometric jitter than tole rable for the grid. The most efficient way to achieve this is probably by m eans of a radial velocity survey. To demonstrate the feasibility of this ap proach, we repeatedly measured the radial velocities for a preselected samp le of 86 nearby Hipparcos K giants with precisions of 5-8 m s(-1). The dist ribution of the intrinsic radial velocity variations for the bona fide sing le K giants shows a maximum around 20 m s(-1), which is small enough not to severely affect the identification of stellar companions around other K gi ants. We use the results of our observations as input parameters for Monte Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with period s mostly larger than 200 years, but only 3.6% of all grid stars would displ ay astrometric jitter larger than 1 mu as. This contamination level is prob ably tolerable.