Astrometric detection of sub-stellar companions with GAIA

Observations with the planned astrometry satellite GAIA are modeled in orde r to determine the detection probabilities for systems consisting of a main -sequence star and a companion of sub-stellar mass. Among the stars within 100 pc from the Sun, most systems having a companion greater than 0.01 sola r masses and with periods between 0.25 and 25 yr will be detected. For thes e systems with periods longer than 10-15 yr, statistical information about orbits is obtained through the curved proper motions. Shorter periods usual ly result in orbital solutions. For nearly all systems with periods less th an five years, accurate orbital elements are obtained from which companion masses can be estimated. For systems having 0.001 solar mass companions, th e detection probability exceeds 0.1 in the 1.2- 10 yr range. Many of these systems will result in orbital solutions.