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We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 fieldultracool dwarfs having spectroscopic and/or kinematic evidence of youth(~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and41 moderate-resolution spectra (R>~750-2000). First, we establish a method forspectral typing M5-L7 dwarfs at near-IR wavelengths that is independent ofgravity. We find that both visual and index-based classification in the near-IRprovide consistent spectral types with optical spectral types, though with asmall systematic offset in the case of visual classification at J and K band.Second, we examine features in the spectra of ~10 Myr ultracool dwarfs todefine a set of gravity-sensitive indices based on FeH, VO, K, Na and H-bandcontinuum shape. We then create an index-based method for classifying thegravities of M6-L5 dwarfs that provides consistent results with gravityclassifications from optical spectroscopy. Our index-based classification candistinguish between young and dusty objects. Guided by the resultingclassifications, we propose a set of low-gravity spectral standards for thenear-IR. Finally, we estimate the ages corresponding to our gravityclassifications.

A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs