Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics
Scientific paper
2012-01-12
Astronomy and Astrophysics
Astrophysics
Solar and Stellar Astrophysics
ApJ, revised. 39 pages of text, 35 figures, 16 tables
Scientific paper
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 79 ultracool dwarfs (spectral types M6--T9) in 45 systems, with a median uncertainty of 1.3 mas (2.5%) while the best are 0.7 mas (1.0%). We provide the first parallaxes for 46 objects in 27 systems, and for another 25 objects in 15 systems, we significantly improve upon published results, with a median (best) improvement of 2.0x (5x). 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 ~0.9 mag and ~0.7 mag, respectively (the Y- and J-band "bumps"), while the H-band flux increases by ~0.0--0.3 mag, and the K- and L'-band fluxes decline 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 J-H = 0.1--0.3 mag, 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.
Dupuy Trent J.
Liu Michael C.
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