First of two. Assigned ID: 346 ----- Attempting Submission 1 (Fri Mar 07 15:01:21 GMT 2008) ----- We propose to obtain from the archive the forty orbits of GTO FGS fringe tracking data secured by the HST Astrometry Science team using FGS 3. These data contain FGS observations of a total of 7 science targets (confirmed members of the Hyades open cluster) and 36 reference stars. These data were previously analyzed and resulted in a parallax for the Hyades published in 1997. Since that time Benedict, McArthur, and Harrison have significantly changed and improved the processing pipeline and the methodology employed to derive parallaxes from HST FGS data. Our expected parallax precision will be nearly three times better than what was previously obtained. These should yield single star absolute magnitudes with errors of 0.04 mag or better, aiding studies of cluster shape and intrinsic main sequence width. Given the importance of the Hyades as a rung in the distance scale ladder, it would be valuable to obtain a definitive, independent parallax for the Hyades and confirm that HIPPARCOS got that one right. A weighted average of the HIPPARCOS and HST parallaxes could provide a valuable test metric for upcoming space astrometry missions such as SIM and/or GAIA.

COOL STARS 12 80000 See Scientific Justification false 0.0 0.0 false false false false false SUPPORTED