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The Transition from L to T: Chemistry and Classification

Published online by Cambridge University Press:  26 May 2016

T. R. Geballe ,
X. Fan ,
D. A. Golimowski ,
G. R. Knapp  and
S. K. Leggett
T. R. Geballe
Affiliation:
Gemini Observatory, 670 N. A 'ohoku Place, Hilo, HI 96720
X. Fan
Affiliation:
Institute for Advanced Study, Olden Lane, Princeton, NJ 08540
D. A. Golimowski
Affiliation:
Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218
G. R. Knapp
Affiliation:
Princeton University Observatory, Princeton, NJ 08540
S. K. Leggett
Affiliation:
Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720

Abstract

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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.

Type
Part 7. Atmospheres and Internal Structure
Information
Symposium - International Astronomical Union , Volume 211: Brown Dwarfs , 2003 , pp. 369 - 376
Copyright
Copyright © Astronomical Society of the Pacific 2001 

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