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一个非常年轻的场L型矮星样本及其对早期褐矮星“锂测试”的意义

A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages

J. Davy Kirkpatrick, Kelle L. Cruz, Travis S. Barman, Adam J. Burgasser, Dagny L. Looper, C. G. Tinney, Christopher R. Gelino, Patrick J. Lowrance, James Lieber… 展开作者

J. Davy Kirkpatrick, Kelle L. Cruz, Travis S. Barman, Adam J. Burgasser, Dagny L. Looper, C. G. Tinney, Christopher R. Gelino, Patrick J. Lowrance, James Liebert, John M. Carpenter, Lynne A. Hillenbrand, John R. Stauffer

备注：
73 pages with 22 figures; to appear in ApJ (Dec 20, 2008, v689n2 issue)
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摘要：

使用大样本的光学光谱的晚型矮星，我们确定了一个子集的晚M到L场矮星，因为存在低重力的功能，在他们的光谱，被认为是异常年轻。从303个L型矮星的组合样本中，我们发现7.6+/-1.6%的矮星年龄小于1亿年。一旦考虑到观察偏差，这个百分比与理论预测一致。我们发现这些年轻的L矮星倾向于落在南半球（Dec < 0 deg），并且可能是以前未被识别的，附近的低质量成员，年轻的协会，如Tucana-Horologium，TW Hydrae，beta Picasso和AB Doradus。我们使用了一个均匀观测的样品，大约有150个6300-10000埃的L和T矮星光谱，这些光谱是用W。M. Keck天文台，以检查6708-A Li I线的强度作为光谱类型的函数，并进一步证实Kirkpatrick等人（2000年）指出的趋势。我们使用我们的低重力光谱调查的强度的Li I线作为年龄的函数。这些数据微弱地表明，对于早期到中期的L型矮星，谱线强度在1亿年前达到最大值，而对于更老的（很少的Gyr）和更年轻的（<1亿年）L型矮星，谱线强度较弱或无法检测到。我们表明，削弱锂在较低的重力模型大气计算预测，现有的观测数据部分证实的效果。需要更大的样本，包含确定年龄的L矮星，以进一步测试这一经验。如果得到证实，这一结果将加强柯克帕特里克等人（2006）首次引用的警告，即在试图确认最年轻的棕矮星的亚恒星性质时，应谨慎使用锂测试。

英文摘要：

Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6+/-1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (Dec < 0 deg) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus. We use a homogeneously observed sample of roughly one hundred and fifty 6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-Resolution Imaging Spectrometer at the W. M. Keck Observatory to examine the strength of the 6708-A Li I line as a function of spectral type and further corroborate the trends noted by Kirkpatrick et al. (2000). We use our low-gravity spectra to investigate the strength of the Li I line as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited in Kirkpatrick et al. (2006) that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

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