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D. A. Boboltz, K. J. Johnston, A. L. Fey, R. A. Gaume, N. Zacharias (U.S. Naval Obs.)
In the SIM Key Science project ``Astrophysics of Reference Frame Tie Objects" we have proposed to investigate the astrophysics of galactic and extragalactic objects which will be used to anchor the SIM astrometric grid and to link the SIM reference frame to the International Celestial Reference Frame (ICRF). Research on extragalactic sources will concentrate on observations of quasars used to establish the SIM/ICRF frame tie to determine the origin of quasar optical emission and to investigate the optical variability and photocenter wander in these objects. Research on galactic objects will focus on the study of galactic frame tie objects with continuum or spectral line radio emission. By studying continuum emitting radio stars (both isolated and those in binary systems) we hope to determine the location of the radio emission with respect to the stellar position(s) as determined by SIM. We will also investigate the transition of Asymptotic Giant Branch (AGB) stars to planetary nebulae by studying maser emitting long-period variables with SIM to distinguish between competing models for envelope asymmetry, to register the position of the masers with respect to the optical photocenter, and to determine the linear size scales for circumstallar envelopes. Research on galactic objects will also take advantage of the data taken with the Full-Sky Astrometric Mapping Explorer (FAME) prior to the launch of SIM. A study of the FAME targets will allow us to identify stars with irregular motions for exclusion from the SIM astrometric grid. Although these ``problem stars" are undesirable for the SIM grid, they will make excellent science targets for further study by SIM. A detailed comparison of the FAME and SIM astrometric grids will allow us to search for any global systematic frame offsets or zonal distortions in the two grids.