L' and M' Photometry of Ultracool Dwarfs
  We have compiled L' and M' photometry of 63 single and binary M, L,and T dwarfs obtained at UKIRT using the MKO filter set. This compilation includes new L' data for 8 L dwarfs and 13 T dwarfs and new M' data for 7 L dwarfs, 5 T dwarfs, and Gl 229A. We compute L_bol, BC_K, and T_eff for 42 dwarfs whose flux-calibrated JHK spectra, L' photometry, and trigonometric parallaxes are available, and we estimate these quantities for 9 other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC_K is a well-behaved function of near-IR spectral type with a dispersion of ~0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T_eff declines steeply and monotonically for types M6-L7 and T4-T9, but is nearly constant at ~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constant T_eff is evidenced by nearly unchanging values of L'-M' between types L6 and T3. We compare the observed K, L', and M' luminosities of L and T dwarfs in our sample with those predicted by model atmospheres with varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M' luminosities of T dwarfs are 1.5-2.5 times larger than those derived empirically for the T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 um by CO, which is not expected under the condition of chemical equilibrium assumed in the models. We determine that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J0423-0414 are probable binary systems. We compute log(L_bol/L_sun) = -5.73 +/- 0.05 and T_eff = 600-750 K for the T9 dwarf 2MASS J0415-0935, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known. 
D. A. Golimowski, S. K. Leggett, M. S. Marley, X. Fan, T. R. Geballe,   G. R. Knapp, F. J. Vrba, A. A. Henden, C. B. Luginbu...