JHK magnitudes for L and T dwarfs and infrared photometric systems

In the last few years a significant population of ultracool L and T dwarfs has been discovered. With effective temperatures ranging from similar to2200 to 700 K, these objects emit most of their radiation in the near-IR, and their spectral energy distributions are dominated by strong molecular absorption bands. These highly structured energy distributions lead to JHK magnitudes that are extremely sensitive to the exact filter bandpass used. In the case of the T dwarfs, the differences between commonly used photometric systems can be as large as 0.4 mag at J and 0.5 mag at J-K. Near-IR magnitudes have been published for L and T dwarfs using a variety of photometric systems. Currently, the data obtained with these systems cannot be accurately compared or combined, as transformations based on the colors of hotter stars are not valid for L and T dwarfs. To address this problem, we have synthesized J, H, and K magnitudes for some of the common photometric systems and present transformation equations with respect to the most atmospheric-independent system, the Mauna Kea Observatory filter set. If the spectral type of the dwarf is known, our transformations allow data to be converted between systems to 0.01 mag, which is better than the typical measurement uncertainty. Transforming on the basis of color alone is more difficult because of the degeneracy and intrinsic scatter in the near-IR colors of L and T dwarfs; in this case J magnitudes can only be transformed to less than or similar to0.05 mag and H and K to less than or similar to0.02 mag.