An Infrared Spectroscopic Sequence of M, L, and T Dwarfs

We present a 0.6-4.1 micron spectroscopic sequence of M, L, and T dwarfs. Thespectra have R~2000 from 0.9 to 2.4 microns and R=2500-200 from 2.9 to 4.1microns. These new data nearly double the number of L and T dwarfs that havereported L-band spectra. The near-infrared spectra are combined with previouslypublished red-optical spectra to extend the wavelength coverage to ~0.6microns. Prominent atomic and molecular absorption features are identifiedincluding neutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weak CH_4absorption features in the H-band spectra of mid- to late-type T dwarfs. Inaddition, we detect for the first time the 0-0 band of the A ^4\Pi - X^4\Sigma^- transition of VO at ~1.06 microns in the spectra of L dwarfs and theP and R branches of the \nu_3 band of CH_4 in the spectrum of a T dwarf. Theequivalent widths of the refractory atomic features all decrease withincreasing spectral type and are absent by a spectral type of ~L0, except forthe 1.189 micron Fe I line which persists to at least ~L3. We compute thebolometric luminosities of the dwarfs in our sample with measured parallaxesand find good agreement with previously published results that use L'-bandphotometry to account for the flux emitted from 2.5 to 3.6 microns. Finally,2MASS J2224381-0158521 (L4.5) has an anomalously red spectrum and the strongest\Delta \nu=+2 CO bands in our sample. This may be indicative of unusually thickcondensate clouds and/or low surface gravity.Comment: 25 pages, 20 figures, 10 tables, accepted for publication in Ap