Interferometric Astrometry of Proxima Centauri and Barnard’s Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions

We report on a sub-stellar companion search utilizing interferometric fringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the Hubble Space Telescope. Our targets were Proxima Centauri and Barnard’s Star. We obtain absolute parallax values for Proxima Cen πabs = 0. 7687± 0. 0003 and for Barnard’s Star πabs = 0. 5454± 0. 0003. 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 Kürster et al. 1999 radial velocity results we exclude all companions with M > 0.8MJup for the range of periods 1 < 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 of Gatewood (1995). Subject headings: astrometry — stars: individual (Proxima Centauri, Barnard’s Star) — stars: late-type — stars: distances — stars: sub-stellar companions Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555 McDonald Observatory, University of Texas, Austin, TX 78712 Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 Astronomy Dept., University of Texas, Austin, TX 78712 Oceanography, University of Rhode Island, Kingston, RI 02881 Lowell Observatory, 1400 West Mars hill Rd., Flagstaff, AZ 86001 Astronomy Dept., Yale University, PO Box 208101, New Haven, CT 06520 Astronomy Dept., University of Virginia, PO Box 3818, Charlottesville, VA 22903 9 Jackson and Tull, Aerospace Engineering Division 7375 Executive Place, Suite 200, Seabrook, Md. 20706 Allied-Signal Aerospace, PO Box 91, Annapolis Junction, MD 20706 Aerospace Engineering, University of Texas, Austin, TX 78712 Environmental Systems Science Center, University of Reading, PO Box 238, Reading RG6 6AL, UK Tracor Aerospace, 6500 Tracor Lane, MD 28-4 Austin, TX 78725