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We present a sequence of high resolution (R~20,000 or 15 km/s) infraredspectra of stars and brown dwarfs spanning spectral types M2.5 to T6.Observations of 16 objects were obtained using eight echelle orders to coverpart of the J-band from 1.165-1.323 micron with NIRSPEC on the Keck IItelescope. By comparing opacity plots and line lists, over 200 weak features inthe J-band are identified with either FeH or H2O transitions. Absorption by FeHattains maximum strength in the mid-L dwarfs, while H2O absorption becomessystematically stronger towards later spectral types. Narrow resolved featuresbroaden markedly after the M to L transition. Our high resolution spectra alsoreveal that the disappearance of neutral Al lines at the boundary between M andL dwarfs is remarkably abrupt, presumably because of the formation of grains.Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed bycondensation. The neutral potassium (K I) doublets that dominate the J-bandhave pressure broadened wings that continue to broaden from ~50 km/s (FWHM) atmid-M to ~500 km/s at mid-T. In contrast however, the measuredpseudo-equivalent widths of these same lines reach a maximum in the mid-Ldwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines withoutextensive pressure-broadened wings, indicative of a lower gravity atmosphere.Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH andH2O features, confirming its status as a rapid rotator. In contrast to otherlate T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absenceof potassium even at high resolution, which may be a metallicity effect or aresult of a cooler, higher-gravity atmosphere.

The NIRSPEC Brown Dwarf Spectroscopic Survey. II. High‐ResolutionJ‐Band Spectra of M, L, and T Dwarfs