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

Abstract

We report on a sub-stellar companion search utilizing interferometric\nfringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the\nHubble Space Telescope. Our targets were Proxima Centauri and Barnard's Star.\nWe obtain absolute parallax values for Proxima Cen pi_{abs} = 0.7687 arcsecond\nand for Barnard's Star pi_{abs} = 0.5454 arcsecond.\n Once low-amplitude instrumental systematic errors are identified and removed,\nour companion detection sensitivity is less than or equal to one Jupiter mass\nfor periods longer than 60 days for Proxima Cen. Between the astrometry and the\nradial velocity results we exclude all companions with M > 0.8M_{Jup} for the\nrange of periods 1 < P < 1000 days. For Barnard's Star our companion detection\nsensitivity is less than or equal to one Jupiter mass for periods long er than\n150 days. Our null results for Barnard's Star are consistent with those of\nGatewood (1995).\n