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Title:
Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program
Authors:
Vrba, F. J.; Henden, A. A.; Luginbuhl, C. B.; Guetter, H. H.; Munn, J. A.; Canzian, B.; Burgasser, A. J.; Kirkpatrick, J. Davy; Fan, X.; Geballe, T. R.; Golimowski, D. A.; Knapp, G. R.; Leggett, S. K.; Schneider, D. P.; Brinkmann, J.
Affiliation:
AA(), AB(Universities Space Research Association.), AC(US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002; , , , , , ), AD(US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002; , , , , , ), AE(US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002; , , , , , ), AF(US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002; , , , , , ), AG(Division of Astronomy and Astrophysics, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095-1562 ), AH(Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, 770 South Wilson Avenue, Pasadena, CA 91125; ), AI(Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721; ), AJ(Gemini Observatory, 670 North A`ohoku Place, Hilo, HI 96720; ), AK(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218; ), AL(Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544; ), AM(UK Infrared Telescope, Joint Astronomy Centre, 660 North A`ohoku Place, Hilo, HI 96720; ), AN(Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802; ), AO(Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349; )
Publication:
The Astronomical Journal, Volume 127, Issue 5, pp. 2948-2968. (AJ Homepage)
Publication Date:
05/2004
Origin:
UCP
AJ Keywords:
Astrometry, Stars: Color-Magnitude Diagrams, Stars: Distances, Stars: Late-Type, Stars: Low-Mass, Brown Dwarfs
DOI:
10.1086/383554
Bibliographic Code:
2004AJ....127.2948V

Abstract

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