ar X iv : a st ro - p h / 04 09 38 9 v 1 1 6 Se p 20 04 1 COOL STARS 13 : SPECTRAL CLASSIFICATION BEYOND

Significant populations of field L and T dwarfs are now known, and we anticipate the discovery of even cooler dwarfs by Spitzer and ground–based infrared surveys. However , as the number of known L and T dwarfs increases so does the range in their observational properties, and difficulties have arisen in interpreting the observations. Although modellers have made significant advances, the complexity of the very low temperature, high pressure, photospheres means that problems remain such as the treatment of grain condensation as well as incomplete and non-equilibrium molecular chemistry. Also, there are several parameters which control the observed spectral energy distribution — effective temperature, grain sedimentation efficiency, metallicity and gravity — and their effects are not well understood. In this paper, based on a splinter session , we discuss classification schemes for L and T dwarfs, their dependency on wavelength, and the effects of the parameters T eff , f sed , [m/H] and logg on optical and infrared spectra. We will also discuss the various hypotheses that have been presented for the transition from the dusty L types to the clear atmosphere T types. We conclude with a brief discussion of the spectral class beyond T. Authors of each Section are identified by their initials. Both optical and infrared classification schemes exist for L dwarfs. In 1999, Kirkpatrick et al. and Martín et al. published optical schemes that use the strength of various features (VO 7912Å, Rb 7948Å, TiO 8432Å, Cs 8521Å, CrH 8611Å) and pseudo–continuum slopes (the color– d 9775/7450 or PC3 8250/7560 flux ratios) to classify L dwarfs. The Martín et al. scheme terminates at L6 and the Kirkpatrick et al. at L8; types derived from the two schemes agree well except for later types, where the Martín scheme produces earlier types than Kirkpatrick. A large number of L dwarfs have been identified by the 2 Micron All Sky Survey (2MASS) and classified on the Kirkpatrick scheme, which has therefore become the commonly used L dwarf classification scheme. Geballe et al. (2002) published an infrared classification scheme for L dwarfs that used a modified color–d index for types L0 to L6, the strength of the 1.5 µm wing of the H 2 O absorption band for L0 to L9, and the shape of the 2.2 µm flux peak (as measured by a CH 4 index) for types L3 to L9. This scheme was pinned to the Kirk-patrick …