Mass Detection Limits for Planetary Companions to Barnard's Star and Proxima Centauri

The \it Hubble Space Telescope Astrometry Science team has collected 52 sets of observations of a star field containing Proxima Centauri and 30 sets of a field containing Barnard's Star. These data are acquired with Fine Guidance Sensor 3 (FGS 3). Our primary goal is to determine upper limits for the masses of planetary companions. Our detection sensitivity is at or below one Jupiter mass for periods between 90 and 600 days for both Proxima Cen and Barnard's Star. Our results for Barnard's Star are consistent with those of Gatewood (1995, Ap&SS, 223, 91). The twenty-eight observation sets of the field containing Barnard's Star have significant value, whether or not we detect perturbations associated with that star. Any systematics introduced by \it HST or FGS 3 should be present in the data for both Barnard's Star and Proxima Cen. We have identified at least one strong systematic effect in these data. We have not, as yet, identified the source. Our detection sensitivity should improve from future re-calibrations and re- reductions. Seven additional observation sets are yet to be obtained for both fields during the remainder of \it HST Cycle 5 (November 1995 - June 1996).