Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the U.S. Naval Observatory (USNO) 1.55–m Strand Astrometric reflector. The results presented here are based on observations obtained between September 2000 and November 2002; about half of the objects have an observational time baseline of ∆t = 1.3 yr and half ∆t = 2.0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the ESO 3.5–m NTT parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing