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

摘要: We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs 18 T measured using the ASTROCAM infrared imager on US Naval Observatory (USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based observations obtained between 2000 September 2002 November; about half of objects have an observational time baseline Δt = 1.3 yr 2.0 yr. Despite these short baselines, astrometric quality is sufficient to produce significant new results, especially nearer dwarfs. Seven in common with USNO optical CCD parallax program control seven European Southern 3.5 New Technology Telescope program. compare both programs. Relative absolute corrections made by employing Two Micron All Sky Survey and/or Sloan Digital photometry reference-frame stars. combine best available California Institute (CIT) system determine MJ, MH, MK values 37 spectral types L0 L8 19 T0.5 T8 selected magnitude versus type color diagrams, results. Luminosities temperatures estimated objects. Of special interest distances several that at or near L-T dwarf boundary so this important transition can be better understood. previously reported early mid luminosity excess clearly confirmed found J, H, K. large number populate luminosity-excess region indicate it cannot due entirely selection effects. sequence extended MJ ≈ 16.9 2MASS J041519-0935, which, d 5.74 pc, least luminous [log(L/L⊙) -5.58] coldest (Teff 760 K) brown known. Combining from paper earlier we find that, contrast dwarfs, there no examples low-velocity (Vtan < 20 km s-1) This consistent study being generally older than briefly discuss future directions astrometry