Spectroscopy of Ultra-Cool Dwarfs

Ultra-cool dwarfs have been defined as dwarfs with spectral types of M7 or later (Kirkpatrick, Henry, & Irwin 1997) and thus include the new L and T spectral classes. The previous IAU report on ultra-cool dwarfs in 1999 described the optical and IR spectral features that characterize these spectral types. In this report, I review the latest progress on the spectral classification of ultra-cool dwarfs, which are divided into two categories: old dwarfs in the field (>1 Gyr) and their young progenitors in the nearest star-forming regions and open clusters (1-100 Myr). Because the former are nearby and the latter are young, these two groups are relatively bright and lend themselves to discovery and detailed study. Technically, the luminosity class of young ultra-cool objects is closer to subgiant than dwarf; nevertheless, I include them in this report on ultra-cool dwarfs.