Parallaxes and Proper Motions of Ultracool Brown Dwarfs of Spectral Types Y and Late T

We present astrometric measurements of 11 nearby ultracool brown dwarfs of spectral types Y and late-T, based on imaging observations from a variety of space-based and ground-based telescopes. These measurements have been used to estimate relative parallaxes and proper motions via maximum likelihood fitting of geometric model curves. To compensate for the modest statistical significance (lsim 7) of our parallax measurements we have employed a novel Bayesian procedure for distance estimation which makes use of an a priori distribution of tangential velocities, V _tan, assumed similar to that implied by previous observations of T dwarfs. Our estimated distances are therefore somewhat dependent on that assumption. Nevertheless, the results have yielded distances for five of our eight Y dwarfs and all three T dwarfs. Estimated distances in all cases are >~ 3 pc. In addition, we have obtained significant estimates of V _tan for two of the Y dwarfs both are <100 km s^-1, consistent with membership in the thin disk population. Comparison of absolute magnitudes with model predictions as a function of color shows that the Y dwarfs are significantly redder in J - H than predicted by a cloud-free model.