Hubble Space Telescope Fine Guidance Sensor Astrometry of the Low-Mass Binary L722-22

Abstract

The M dwarf star L722-22 (LHS 1047, GJ 1005) was discovered to be a binary in 1979. Analysis of ground-based data indicated a mass near 0.06 M_⊙ for the secondary star, well below the nominal stellar mass limit of 0.08 M_⊙. The close, faint binary was near the limit for ground-based astrometry and was approved for Hubble Space Telescope Fine Guidance Sensor (FGS) observations in 1992. The relative orbital motion of the binary has been monitored using FGS ``transfer'' mode measurements. The trigonometric parallax and motion of the primary about the center of mass were determined from the FGS ``position'' mode observations. All possible background reference stars in the FGS field-of-view were used. The relative orbit and fractional masses have been determined with far higher precision and accuracy than possible with ground-based techniques for this close, faint binary. The FGS observations definitely eliminate the possibility that the secondary star is a candidate for having a substellar mass, and place its mass and lower mass error range well above the stellar mass limit. Masses of 0.179 and 0.112 M_⊙ have been found for the two components, with formal random errors as low as 1.5%. The mass errors resulting from the correction from relative to absolute parallax are somewhat larger.