A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs

We present a near-infrared (0.9–2.4 μm) spectroscopic study of 73 field ultracool dwarfs having spectroscopic and/or kinematic evidence of youth (≈10–300 Myr). Our sample is composed of 48 low-resolution ( R ≈ 100) spectra and 41 moderate-resolution spectra ( R ≳ 750–2000). First, we establish a method for spectral typing M5–L7 dwarfs at near-IR wavelengths that is independent of gravity. We find that both visual and index-based classification in the near-IR provides consistent spectral types with optical spectral types, though with a small 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 to define a set of gravity-sensitive indices based on FeH, VO, K i , Na i , and H -band continuum shape. We then create an index-based method for classifying the gravities of M6–L5 dwarfs that provides consistent results with gravity classifications from optical spectroscopy. Our index-based classification can distinguish between young and dusty objects. Guided by the resulting classifications, we propose a set of low-gravity spectral standards for the near-IR. Finally, we estimate the ages corresponding to our gravity classifications.