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Paper: Detecting Faint Companions with SIM
Volume: 194, Working on the Fringe: Optical and IR Interferometry from Ground and Space
Page: 78
Authors: Liu, Y.; Peterson, D. M.
Abstract: The defining problem for the Space Interferometry Mission (SIM) is to establish a set of grid stars against which all other position measurements are performed. Expecting that there will remain a serious problem with contamination from undetected companions, we are studying the applicability of the CID method (Wielen 1996) where the secondary could be detected by a color dependent shift in the measured position. If this can be done efficiently it might be possible to flush out problem grid members in the initial phases of the mission, saving observing time and minimizing the potential loss of certain regions of the sky from the astrometric solution. In looking at this method, we found that the photo-center positions of binaries as measured by SIM do not necessarily lie on a straight line as the light ratio varies. The having the measured photo-centers lie on a straight line joining the two stars is valid only in the case where separations are small compared to the width of the central fringe. As a result we will not be able to implement a statistical test based on that simple model. We therefore look at a more general, 2-dimensional statistical test to search for companions. To test it's capabilities we consider three possible scenarios, G0V primary plus lower mass main sequence companions, K2 giant (Pop I) plus main sequence companions, and weak line K2 giant plus weakline subdwarfs companions, as representatives of the problem. At 107 detected photons (a 4 μ as measurement), we are able to see a secondary fainter by 4 magnitudes at separations of 0.6 milli-arcsec (mas) or greater at 95% confidence. At a magnitude difference of 2, separations down to 0.3 mas will allow detection of the secondary. These results apply only to companions that are within the central fringe defined by the primary. We will next extend this analysis to include companions that lie several fringes away, a circumstance that should provide detection to fainter limiting magnitudes.
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