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 Gl 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (Teff ≲ 2400 K) for which L′ and M′ measurements have been reported. We compute Lbol, BCK, and Teff 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. BCK 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. Teff 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 Teff 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 Teff. 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 ML′ and MM′ 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 (Lbol/L⊙) = -5.73 ± 0.05 and Teff = 600–750 K for the T9 dwarf 2MASSI J0415195-093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.