Interferometric Astrometry of Proxima Centauri and Barnard's Star Using [ITAL]HUBBLE SPACE TELESCOPE[/ITAL] Fine Guidance Sensor 3: Detection Limits for Substellar Companions

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 Barnard's star. We obtain absolute parallax values of πabs = 07687 ± 00003 for Proxima Cen and πabs = 05454 ± 00003 for Barnard's star. Once low-amplitude instrumental systematic errors are identified 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 Kürster et al., we exclude all companions with M > 0.8MJup for the range of periods 1 day < P < 1000 days. For Barnard's star, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 150 days. Our null results for Barnard's star are consistent with those reported by Gatewood in 1995.