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.
- Publication:
-
The Astrophysical Journal Supplement Series
- Pub Date:
- August 2012
- DOI:
- arXiv:
- arXiv:1201.2465
- Bibcode:
- 2012ApJS..201...19D
- Keywords:
-
- astrometry;
- brown dwarfs;
- infrared: stars;
- parallaxes;
- proper motions;
- stars: low-mass;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Instrumentation and Methods for Astrophysics
- E-Print:
- Accepted to ApJ. New arXiv posting includes 4 new parallaxes and an overall improvement in precision of 1.3x thanks to additional CFHT astrometry for many targets. All data compiled in this paper (and more) are available online: http://www.cfa.harvard.edu/~tdupuy/plx