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R.A. Wade, M.A. Stark (Penn State)
Wielen (1996) coined the term ``variability-induced mover'' (VIM) to denote a binary star whose photocenter shows an astrometric shift that is caused by a temporary change in the luminosity ratio of the component stars. It has been suggested (S.\ Frink, private comm.) that the VIM phenomenon may be detrimental to the Space Interferometry Mission (SIM), by destroying the astrometric stability of the Grid Star network that will be used by to establish SIM's reference frame. The Grid will be composed of about 1300 halo giant stars with typical distances of a few kpc. In some cases, late-type main sequence companions orbiting these giants with periods of 102--103 yr will survive screening for radial velocity variations and other duplicity tests. The angular separations of these binaries will be ~104 \muas. A fraction of the ``certified'' Grid stars will then be VIMs, owing either to mmag variations of the halo giant itself or to variations of its companion star. If VIMs are present in the Grid with sufficient amplitudes (jitter greater than a few \muas), varying on relevant timescales and in sufficiently many cases, the robustness of the Grid as a whole may be undermined.
This contribution presents an independent assessment of the importance of this effect for the SIM astrometric Grid.
Supported by NASA.