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Published December 20, 2008 | Published
Journal Article Open

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


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 (decl: < 0°) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, β Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength 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 x 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 by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

Additional Information

© 2008. The American Astronomical Society. Received 2008 May 22; accepted 2008 August 21. We would like to thank Peter Allen for useful discussions on his 2005 paper. We would like to thank the staff at the W.M. Keck Observatory for their help in acquiring the LRIS data: observing assistants Joel Aycock, Gary Puniwai, Julie Rivera, Gabrelle Saurage, and Cynthia Wilburn as well as instrument specialists Paola Amico, Randy Campbell, Bob Goodrich, Grant Hill, Marc Kassis, and Greg Wirth. We would also like to thank the staff at the Subaru Telescope for their guidance in taking and reducing the FOCAS spectra: telescope operators Alan Hatakeyama and Robert Potter along with FOCAS support astronomer Takashi Hattori. We further wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The finder charts in Figure 1 were generating using the mosaicking service available at http://hachi.ipac.caltech.edu:8080/montage/. Our research has benefitted from the M, L, and T dwarf compendium housed at DwarfArchives.org whose server was funded by a NASA Small Research Grant, administered by the American Astronomical Society.

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