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Paper: Testing Hipparcos K Giants as Grid Stars for SIM
Volume: 194, Working on the Fringe: Optical and IR Interferometry from Ground and Space
Page: 128
Authors: Frink, S.; Quirrenbach, A.; Röser, S.; Schilbach, E.
Abstract: We investigate several parameters of Hipparcos K giants with the aim to explore their suitability as SIM (Space Interferometry Mission) grid stars. K giants can easily be identified in the Hipparcos Catalogue (ESA 1997) using the spectral information as well as photometry and parallaxes provided there to discriminate them from K dwarfs. The overall number of K giants in the Hipparcos Catalogue is about 32500. In order to qualify as SIM grid objects, the stars have to fulfill several criteria. First, there have to be enough objects (3000-4000) which should be distributed uniformly over the sky. Second, and most important, there should be no indication for an unknown companion, and ideally not even a planet, because the orbital motion would affect the desired microarcsecond accuracy of the grid. In order to sort out such unwanted possibly double or multiple systems among the HIP K giants, the various flags in the Hipparcos and Tycho Catalogues indicating problems with the astrometric solution can serve as a discrimination criterion. Special attention should be paid to differences between the instantaneous proper motions (as measured by Hipparcos) and the proper motions averaged over a larger epoch of about 80 years as provided by the TRC (Tycho Reference Catalogue, Hog et al. 1998) or the ACT (The ACT Reference Catalog, Urban et al. 1997), since these are likely to reflect orbital motions due to companions. The corresponding objects clearly should be avoided as SIM grid stars.
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