Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program
Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program
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美国海军天文台红外天体测量项目 40 个 L 型和 T 型矮星的初步视差
DOI：
10.1086/383554
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
J. Brinkmann
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文献类型：
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作者：
F. Vrba;A. Henden;C. Luginbuhl;H. Guetter;J. Munn;B. Canzian;A. Burgasser;J. Kirkpatrick;Xiaohui Fan;T. Geballe;D. Golimowski;G. Knapp;S. Leggett;D. P. Schneider;J. Brinkmann
本文介绍了利用美国海军天文台（USNO） 1.55 m Strand天文反射器上的ASTROCAM红外成像仪测量的22个L矮星和18个T矮星的初步三角视差和固有运动。这里给出的结果是基于2000年9月至2002年11月的观测结果；大约一半的天体的观测时间基线为Δt = 1.3年，另一半的观测时间基线为Δt = 2.0年。尽管这些时间基线很短，但天文测量的质量足以产生重要的新结果，特别是对于较近的T矮星。为了质量控制，USNO光学CCD视差项目有7个共同的对象，欧洲南方天文台3.5米新技术望远镜视差项目有7个共同的对象。我们比较了这两个项目的天体测量质量。相对绝对视差校正是通过使用2微米全巡天和/或斯隆数字巡天光度法对参考框架恒星进行的。我们将USNO红外和光学视差与加州理工学院（CIT）系统测光技术相结合，测定了37颗光谱类型为L0和L8的L型矮星和19颗光谱类型为T0.5和T8的T型矮星的MJ、MH和MK值，并根据这些结果给出了选定的绝对星等与光谱类型和颜色图。对这些天体的光度和温度进行了估计。特别令人感兴趣的是位于或接近L-T矮星边界的几个物体的距离，以便更好地理解这一重要的转变。先前报道的早期到中期的T矮星亮度过剩在J、H和k处得到了明确的证实和发现。大量的天体占据了这个亮度过剩区域，这表明它不可能完全是由于选择效应。T矮星序列被2MASS J041519-0935扩展到MJ≈16.9，它在d = 5.74 pc时被发现是已知最不亮的褐矮星[log(L/L⊙)= -5.58]和最冷的褐矮星（Teff≈760 K）。将本文的结果与早先的USNO CCD结果相结合，我们发现，与L矮星相比，没有低速（van < 20 km s-1） T矮星的例子。这与本研究中的T型矮星普遍比L型矮星年龄大一致。我们简要讨论了美国国家天文台红外天文测量计划的未来发展方向。
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/L⊙) = -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.