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B.D. Mason, A.R. Hajian, S.E. Urban (U.S. Naval Observatory)
The USNO is preparing an input catalog of SIM grid stars by utilizing existing databases, observing capabilities, and instrumental development. The SIM grid should be astrometrically stable at the 4 \muas level over the five year life of the program. Therefore, multiple stars must be avoided.
Many programs for Grid Star selection have focused on distant, faint stars (for example, 11 to 13th magnitude K giants) because companions to these would cause less apparent ``wobble'' than those around nearby stars. This argument, at first glance, seems logical. However, there is a suprisingly large range of masses and periods can make stars even at distances of 2 Kpc unstable at 4 \muas. Furthermore, due to their apparent magnitude, these stars cannot be surveyed for companions.
Another option is to use nearby, bright star stars for the grid. One of most compelling reasons to use them is the extent to which observations can be made from the ground before SIM launch to maximize the likelyhood of astrometric stability of the grid. While nearby stars require multiple, overlapping techniques to thoroughly investigate the angular separations where companions can cause astrometric motion > 4\muas, K Giants can not be adequately investigated from the ground prior to SIM launch. If K Giants are selected for the astrometric grid we may learn a lot about the multiplicity characteristics of a large sample of these stars, however, other scientific missions of SIM will be in peril.
Stars in the apparent magnitude range 6.5\,<\,mv\,<\,8.5 with spectral types from A5V to G5V are being considered. Due to their proximity, virtually all regimes of separation can be excluded for stellar companions except for very short period, low inclination systems, or slightly wider, high \Deltam systems. For shorter-period systems, substellar companions can be avoided as well.
This work has been supported by NASA and the SIM preparatory science program through NRA 98-OSS-007. We gratefully acknowledge this support.