Near-infrared photometry and spectroscopy of L and T dwarfs: The effects of clouds, gravity, and effective temperature

We present new JHKL′M′ photometry on the MKO system for a large sample of L and T dwarfs identified from SDSS and 2MASS and classified according to the scheme of Geballe et al. (2002). We have compiled a sample of 105 L and T dwarfs that are uniformly classified and measured on a single photometric system. The scattered JHK spectral indices and colors of L dwarfs are likely caused by variations in the altitudes, distributions, and thicknesses of condensate clouds. Scatter in the H-K colors of late T dwarfs probably reflects the sensitivity of the K-band flux to pressure induced H ₂ opacity, which itself is sensitive to surface gravity. The M′ luminosities of late-T dwarfs are 1.5-2.5 times fainter than predicted under conditions of chemical equilibrium. We have computed L _bol and T _eff for 42 L and T dwarfs whose trigonometric parallaxes have been measured. We find that T _eff ≈1450 K for types L7-T4, which supports recent models that attribute the changing JH K-band luminosities and spectral features across the L-T transition to rapid changes in the condensate clouds over a narrow range of T _eff. We compute T _eff = 600-750 K for 2MASS 0415-0935 (T9), which supplants Gl 570D as the coolest known brown dwarf.