THE HAWAII INFRARED PARALLAX PROGRAM. I. ULTRACOOL BINARIES AND THE L/T TRANSITION ,

Abstract

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 1.7 times (5 times). Three systems show astrometric perturbations indicative of orbital motion; two are known binaries (2MASS J0518−2828AB and 2MASS J1404−3159AB) and one is spectrally peculiar (SDSS J0805+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 ≈L8 to ≈T4.5, flux in the Y and J bands increases by ≈0.7 mag and ≈0.5 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 μm. 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 ( J − H ) MKO = 0.1–0.3 mag, ( J − K ) MKO = 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.