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Abstract: Using a large sample of optical spectra of late-type dwarfs, we identify asubset of late-M through L field dwarfs that, because of the presence oflow-gravity features in their spectra, are believed to be unusually young. Froma combined sample of 303 field L dwarfs, we find observationally that7.6+-1.6% are younger than 100 Myr. This percentage is in agreement withtheoretical predictions once observing biases are taken into account. We findthat these young L dwarfs tend to fall in the southern hemisphere Dec < 0 degand may be previously unrecognized, low-mass members of nearby, youngassociations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus.We use a homogeneously observed sample of roughly one hundred and fifty6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-ResolutionImaging Spectrometer at the W. M. Keck Observatory to examine the strength ofthe 6708-A Li I line as a function of spectral type and further corroborate thetrends noted by Kirkpatrick et al. 2000. We use our low-gravity spectra toinvestigate the strength of the Li I line as a function of age. The data weaklysuggest that for early- to mid-L dwarfs the line strength reaches a maximum fora few 100 Myr, whereas for much older few Gyr and much younger <100 Myr Ldwarfs the line is weaker or undetectable. We show that a weakening of lithiumat lower gravities is predicted by model atmosphere calculations, an effectpartially corroborated by existing observational data. Larger samplescontaining L dwarfs of well determined ages are needed to further test thisempirically. If verified, this result would reinforce the caveat first cited inKirkpatrick et al. 2006 that the lithium test should be used with cautionwhen attempting to confirm the substellar nature of the youngest brown dwarfs.

Autor: J. Davy Kirkpatrick, Kelle L. Cruz, Travis S. Barman, Adam J. Burgasser, Dagny L. Looper, C. G. Tinney, Christopher R. Gelino, Pa

Fuente: https://arxiv.org/

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