Interferometric Astrometry of the Low-Mass Binary GL 791.2 (= HU Del) UsingHubble Space Telescope Fine Guidance Sensor 3: Parallax and Component Masses

Abstract

With fourteen epochs of fringe-tracking data spanning 1.7 yr from Fine Guidance Sensor 3 we have obtained a parallax (π_abs=113.1+/-0.3 mas) and perturbation orbit for Gl 791.2A. Contemporaneous fringe-scanning observations yield only three clear detections of the secondary on both interferometer axes. They provide a mean component magnitude difference, ΔV=3.27+/-0.10. The period (P=1.4731 yr), from the perturbation orbit, and the semimajor axis (a=0.963+/-0.007 AU), from the measured component separations with our parallax, provide a total system mass M_A+M_B=0.412+/-0.009 M_solar. Component masses are M_A=0.286+/-0.006 M_solar and M_B=0.126+/-0.003 M_solar. Gl 791.2A and Gl 791.2B are placed in a sparsely populated region of the lower main-sequence mass-luminosity relation where they help define the relation because the masses have been determined to high accuracy, with errors of only 2%. 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.