J-band infrared spectroscopy of a sample of brown dwarfs using NIRSPEC on Keck II

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L 2 to L8/9, and che seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSP EC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving powe r of R approximate to 2500 from 1.135 to 1.360 mu m (approximately J band) are presented for each source. At this resolution, a rich spectral structur e is revealed, much of which is due to blending of unresolved molecular tra nsitions. Strong lines due to neutral potassium (K (I)) and bands due to ir on hydride (FeH) and steam (H2O) change significantly throughout the L sequ ence. Iron hydride disappears between L5 and L8, the steam bands deepen, an d the K (I) lines gradually become weaker but wider because of pressure bro adening. An unidentified feature occurs at 1.22 mu m that has a temperature dependence like FeH but has no counterpart in the available FeH opacity da ta. Because these objects are 3-6 mag brighter in the near-infrared compare d with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523 + 3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantl y different from the methane-dominated objects such as Gl 229B or SDSS 1624 +0029.