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 Space Telescope. Our targets were Proxima Centauri and Barnard's star. We obtain absolute parallax values of πabs = 0.″7687 ± 0.″0003 for Proxima Cen and πabs = 0.″5454 ± 0.″0003 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.
A Mass for the Extrasolar Planet Gliese 876b Determined from Hubble Space Telescope Fine Guidance Sensor 3 Astrometry and High-Precision Radial Velocities
We report the first astrometrically determined mass of an extrasolar planet, a companion previously detected by Doppler spectroscopy. Radial velocities first provided an ephemeris with which to schedule a significant fraction of the Hubble Space Telescope (HST) observations near companion peri- and apastron. The astrometry residuals at these orbital phases exhibit a systematic deviation consistent with a perturbation due to a planetary mass companion. Combining HST astrometry with radial velocities, we solve for the proper motion, parallax, perturbation size, inclination, and position angle of the line of nodes, while constraining period, velocity amplitude, longitude of periastron, and eccentricity to values determined from radial velocities. We find a perturbation semimajor axis and inclination, α = 0.25 ± 0.06 mas, i = 84° ± 6°, and Gl 876 absolute parallax, πabs = 214.6 ± 0.2 mas. Assuming that the mass of the primary star is M* = 0.32 M☉, we find the mass of the planet, Gl 876b, Mb = 1.89 ± 0.34 MJup.
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