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 TelescopeÏs 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 Ðrst evidence of Gl 105CÏs orbital motion. Previous longterm astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105AÏs perturbation is D60 yr. To satisfy both the observed orbital motion and Gl 105AÏs astrometric period, Gl 105CÏs orbit must have an eccentricity of D0.75 and a semimajor axis of D15 AU. Measurements of Gl 105AÏs 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.