Interferometric Astrometry of the Low-Mass Binary Gl 791.2 (= HU Del) Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Parallax and Component Masses

With fourteen epochs of fringe-tracking data spanning 1.7 yr from Fine Guidance Sensor 3 we have obtained a parallax (πabs = 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 MA + MB = 0.412 ± 0.009 M⊙. Component masses are MA = 0.286 ± 0.006 M⊙ and MB = 0.126 ± 0.003 M⊙. 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%.