The Very Low Mass Component of the Gliese 105 System

Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and its very low mass companion Gliese 105C have been obtained using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and near-infrared camera and multiobject spectrometer (NICMOS). The optical and near-infrared colors of Gl 105C strongly suggest a spectral type of M7 V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl 105C's orbital motion. Previous long-term astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105A's perturbation is ~60 yr. To satisfy both the observed orbital motion and Gl 105A's astrometric period, Gl 105C's orbit must have an eccentricity of ~0.75 and a semimajor axis of ~15 AU. Measurements of Gl 105A's radial velocity over 12 yr show a linear trend with a slope of 11.3 m s^-1 yr^-1, which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to Gl 105A have been detected, we conclude that Gl 105C is probably the source of the 60 yr astrometric perturbation.