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

TL;DR

This study measures the distances and motions of ultracool brown dwarfs of spectral types Y and late-T, using a novel Bayesian approach to estimate their parallaxes and tangential velocities, revealing their properties and comparison with models.

Contribution

Introduces a Bayesian method for estimating distances of ultracool brown dwarfs with low-significance parallaxes, improving understanding of their kinematics and properties.

Findings

Distances for five Y and all three T dwarfs are > 3 pc.

Two Y dwarfs have tangential velocities < 100 km/s, indicating thin disk membership.

Y dwarfs are redder in J-H than cloud-free model predictions.

Abstract

We present astrometric measurements of eleven 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 (<~ 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, Vtan, 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 Vtan for two of the Y dwarfs; both are <100 km/s, 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