Interferometric Astrometry of Proxima Centauri and Barnardïs Star Using Hubbl E Space T El Escope Fine Guidance Sensor 3: Detection Limits for Substellar Companions1

We report on a substellar-companion search utilizing interferometric fringe-tracking astrometry acquired with Fine Guidance Sensor 3 on the Hubble Space Telescope. Our targets were Proxima Centauri and BarnardÏs star. We obtain absolute parallax values of for Proxima nabs\ 0A.7687^ 0A.0003 Cen and for BarnardÏs star. Once low-amplitude instrumental systematic errors nabs\ 0A.5454^ 0A.0003 are identiÐed and removed, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 60 days for Proxima Cen. Between the astrometry and the recent radial velocity results of et al., we exclude all companions with for the range of periods 1 KuŽ rster M [ 0.8MJup day \ P\ 1000 days. For BarnardÏs star, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 150 days. Our null results for BarnardÏs star are consistent with those reported by Gatewood in 1995.