L′ and M′ photometry of ultracool dwarfs

We have compiled L′ (3.4-4.1 μm) and M′ (4.6-4.8 μm) photometry of 63 single and binary M, L, and T dwarfs obtained at the United Kingdom Infrared Telescope using the Mauna Kea Observatory filter set. This compilation includes new L′ measurements of eight L dwarfs and 13 T dwarfs and new M′ measurements of seven L dwarfs, five T dwarfs, and the M1 dwarf G1 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (T _eff ≲ 2400 K) for which L′ and M′ measurements have been reported. 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 nine other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC _K is a well-behaved function of near-infrared 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 it 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. It also supports recent models that attribute the changing near-infrared luminosities and spectral features across the L-T transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of J _eff. The L′ and M′ luminosities of early-T dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6-T5 to assert that M _L′ and M _M′ are strictly monotonic within this range of types. We compare the observed K, L′, and M′ luminosities of L and T dwarfs in our sample with those predicted by precipitating-cloud and cloud-free models for 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 late-T dwarfs are 1.5-2.5 times larger than those derived empirically for the late-T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 μm by CO, which is not expected under the condition of thermochemical equilibrium assumed in the models. Our photometry and bolometric calculations indicate that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57-041403.5 are probable binary systems. We compute log (L _bol/L _⊙) = -5.73 ± 0.05 and T _eff= 600-750 K for the T9 dwarf 2MASSI J0415195-093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.