L' and M' Photometry of Ultracool Dwarfs
Abstract
We have compiled L' (3.44.1 μm) and M' (4.64.8 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 (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 fluxcalibrated JHK spectra, L' photometry, and trigonometric parallaxes are available, and we estimate these quantities for nine other dwarfs whose parallaxes and fluxcalibrated spectra have been obtained. BC_{K} is a wellbehaved function of nearinfrared spectral type with a dispersion of ~0.1 mag for types M6T5 it is significantly more scattered for types T5T9. T_{eff} declines steeply and monotonically for types M6L7 and T4T9, but it is nearly constant at ~1450 K for types L7T4 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 nearinfrared luminosities and spectral features across the LT transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of T_{eff}. The L' and M' luminosities of earlyT dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6T5 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 precipitatingcloud and cloudfree models for varying surface gravities and sedimentation efficiencies. The models indicate that the L3T4.5 dwarfs generally have higher gravities (logg=5.05.5) than the T6T9 dwarfs (logg=4.55.0). The predicted M' luminosities of lateT dwarfs are 1.52.5 times larger than those derived empirically for the lateT dwarfs in our sample. This discrepancy is attributed to absorption at 4.54.9 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 Kelu1 and the T0 dwarf SDSS J042348.57041403.5 are probable binary systems. We compute log(L_{bol}/L_{solar})=5.73+/0.05 and T_{eff}=600750 K for the T9 dwarf 2MASSI J0415195093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.
 Publication:

The Astronomical Journal
 Pub Date:
 June 2004
 DOI:
 10.1086/420709
 arXiv:
 arXiv:astroph/0402475
 Bibcode:
 2004AJ....127.3516G
 Keywords:

 Stars: Fundamental Parameters;
 Stars: LateType;
 Stars: LowMass;
 Brown Dwarfs;
 Astrophysics
 EPrint:
 50 pages, including 10 figures and 6 tables, to be published in June 2004 issue of The Astronomical Journal. Revised 25 Mar 2003: Removed footnote 17