The NIRSPEC Brown Dwarf Spectroscopic Survey. II. High-Resolution J-Band Spectra of M, L, and T Dwarfs^*

We present a sequence of high-resolution (R ~ 20,000, or 15 km s^-1) infrared spectra of stars and brown dwarfs spanning spectral types M2.5 to T6. Observations of 16 objects were obtained using eight echelle orders to cover part of the J band from 1.165-1.323 μm with NIRSPEC on the Keck II telescope. By comparing opacity plots and line lists, over 200 weak features in the J band are identified with either FeH or H₂O transitions. Absorption by FeH attains maximum strength in the mid-L dwarfs, while H₂O absorption becomes systematically stronger toward later spectral types. Narrow resolved features broaden markedly after the M to L transition. Our high-resolution spectra also reveal that the disappearance of neutral Al lines at the boundary between M and L 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 by condensation. The neutral potassium (K I) doublets that dominate the J band have pressure-broadened wings that continue to broaden from ~50 km s^-1 (FWHM) at mid M to ~500 km s^-1 at mid T. In contrast, however, the measured pseudo-equivalent widths of these same lines reach a maximum in the mid-L dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without extensive pressure-broadened wings, indicative of a lower gravity atmosphere. Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and H₂O features, confirming its status as a rapid rotator. In contrast to other late-T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence of potassium even at high resolution, which may be a metallicity effect or a result of a cooler, higher gravity atmosphere.