The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T   Transition
  We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17 systems, we significantly improve upon published results, with a median (best) improvement of 1.7x (5x). Three systems show astrometric perturbations indicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and 2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). In addition, we present here a large set of Keck adaptive optics imaging that more than triples the number of binaries with L6--T5 components that have both multi-band photometry and distances. Our data enable an unprecedented look at the photometric properties of brown dwarfs as they cool through the L/T transition. Going from \approxL8 to \approxT4.5, flux in the Y and J bands increases by \approx0.7 mag and \approx0.5 mag, respectively (the Y- and J-band "bumps"), while flux in the H, K, and L' bands declines monotonically. This wavelength dependence is consistent with cloud clearing over a narrow range of temperature, since condensate opacity is expected to dominate at 1.0--1.3 micron. Interestingly, despite more than doubling the near-IR census of L/T transition objects, we find a conspicuous paucity of objects on the color--magnitude diagram just blueward of the late-L/early-T sequence. This "L/T gap" occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, and implies that the last phases of cloud evolution occur rapidly. Finally, we provide a comprehensive update to the absolute magnitudes of ultracool dwarfs as a function of spectral type using a combined sample of 314 objects. 
Trent J. Dupuy (CfA/SAO) and Michael C. Liu (IfA/Hawaii)
astro-ph.SR astro-ph.IM