Interferometric astrometry of Proxima Centauri and Barnard's star using Hubble Space Telescope 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 Spa ce Telescope. Our targets were Proxima Centauri and Barnard's star. We obta in absolute parallax values of pi(abs) = 0." 7687 +/- 0." 0003 for Proxima Cen and pi(abs) = 0." 5454 +/- 0." 0003 for Barnard's star. Once low-amplit ude instrumental systematic errors are identified and removed, our companio n detection sensitivity is less than or equal to one Jupiter mass for perio ds longer than 60 days for Proxima Cen. Between the astrometry and the rece nt radial velocity results of Kurster et al., we exclude all companions wit h M > 0.8M(Jup) 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 J upiter mass for periods longer than 150 days. Our null results for Barnard' s star are consistent with those reported by Gatewood in 1995.