The Transition from L to T: Chemistry and Classification
T. R. Geballe
Gemini Observatory, 670 N. A 'ohoku Place, Hilo, HI 96720
X. Fan
Institute for Advanced Study, Olden Lane, Princeton, NJ 08540
D. A. Golimowski
Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218
G. R. Knapp
Princeton University Observatory, Princeton, NJ 08540
S. K. Leggett
Joint Astronomy Centre, 660 N. A 'ohoku Place, Hilo, HI 96720
Abstract.
The growing numbers of brown dwarfs have largely been divided into two new spectral classes, L and T. Subclassification systems for L, based on optical (0.6-1.0 µm) spectra, have existed since 1999, but there is a need for infrared systems for both spectral classes. The differences in the infrared (1.0-2.5 µm) characteristics of L spectra and T spectra observed prior to 2000 are huge, all then-known T dwarfs showing strong methane absorption bands, and all L dwarfs devoid of these bands. However, in the last 2-3 years a significant number of brown dwarfs with infrared spectra in transition between the L and first observed T types have been discovered. The spectra of late L dwarfs and these "transition objects" can be put into a well ordered sequence of increasing H2O and CH4 band strengths and decreasing CO band strengths. Specific infrared spectral indicators have been found that allow the L and T sequences to be linked and the boundary between L and T to be precisely defined. In current T classification schemes the transition objects are defined as early T dwarfs. We recommend that the boundary between the L and T classes be defined as the first appearance of methane absorption just longward of 1.60 µm, in the H band.
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