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 Hubbl e Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and near-infrared camera and multiobject spectrometer (NICMOS). The optical and near-infrare d colors of GI 105C strongly suggest a spectral type of M7 V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of GI 105C's orbital motion. Previous longterm astrometric studies at Sprou l and McCormick Observatories have shown that the period of G1 105A's pertu rbation is similar to 60 yr. To satisfy both the observed orbital motion an d GI 105A's astrometric period, GI 105C's orbit must have an eccentricity o f similar to 0.75 and a semimajor axis of similar to 15 AU. Measurements of GI 105A's radial velocity over 12 yr show a linear trend with a slope of 1 1.3 m s(-1) yr(-1), which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to G1 105A have been d etected, we conclude that GI 105C is probably the source of the 60 yr astro metric perturbation.