Very Cool L-Dwarfs

Abstract

ASTRONOMY Dwarfs are low mass, very cool, main-sequence stars, and recent large-scale star surveys have identified a new spectral class, called the L-dwarfs. The low estimated masses of L-dwarfs suggest that most of these objects are brown dwarfs that are rich in hydrides rather than methane. McLean et al. have obtained high-resolution spectra of six L-dwarfs that range in temperature from 2200-2000 K to 1600-1250 K with the near-infrared cryogenic spectrograph (NIRSPEC) on the Keck II telescope. As the temperature of the star decreases, the spectrum changes systematically: the neutral potassium band broadens and weakens, but persists in the lowest temperature L-dwarf; the iron hydride (FeH) band weakens and disappears at about 1600 K; the water band strengthens, as do the neutral alkali metal bands of Na, Rb, and Cs. These spectral features are consistent with models of a cooling atmosphere in which the iron hydride condenses into iron metal. The less refractory alkali metals can persist as neutral species to lower temperatures than the iron, and because they are present in the L-dwarf spectra to about 1200 K, they will form sulfides rather than chlorides when they do condense.— LR Astrophys. J. 533 , L45 (2000).