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.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.