TWO EXTRAORDINARY SUBSTELLAR BINARIES AT THE T/Y TRANSITION AND THE Y-BAND FLUXES OF THE COOLEST BROWN DWARFS

• Published in: The Astrophysical journal Vol. 758; no. 1; pp. 1 - 15
• Main Authors: LIU, Michael C, DUPUY, Trent J, BOWLER, Brendan P, LEGGETT, S. K, BEST, William M. J
• Format: Journal Article
• Language: English
• Published: Bristol IOP 10-10-2012
• Subjects: ABSORPTION; Adaptive optics; Approximation; ASTRONOMY; ASTROPHYSICS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; Binary systems; Brown dwarf stars; COLOR; DWARF STARS; Earth, ocean, space; Exact sciences and technology; Fluxes; GRAVITATION; INFRARED SPECTRA; LASERS; MASS; NEAR INFRARED RADIATION; OPTICS; PHOTOMETRY; PHOTOSPHERE; RESOLUTION; Spectra; SURFACES; VARIATIONS; infrared: stars; Color; Spectral type; binaries: close; Infrared spectra; Secondary component; Artificial guide stars; brown dwarfs; Metallicity; Brown dwarf stars; Surface gravity; Mass ratio; Close binary stars; techniques: high angular resolution; Adaptive optics; binaries: general; Infrared star; Age; Optical spectrum
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/758/1/57

Using Keck laser guide star adaptive optics imaging, we have found that the T9 dwarf WISE J1217+1626 and T8 dwarf WISE J1711+3500 are exceptional binaries, with unusually wide separations ([approximate]0".8, 8-15 AU), large near-IR flux ratios ([approximate]2-3 mag), and small mass ratios ([approximate]0.5) compared to previously known field ultracool binaries. Keck/NIRSPEC H-band spectra give a spectral type of Y0 for WISE J1217+1626B, and photometric estimates suggest T9.5 for WISE J1711+3500B. The WISE J1217+1626AB system is very similar to the T9+Y0 binary CFBDSIR J1458+1013AB; these two systems are the coldest known substellar multiples, having secondary components of [approximate]400 K and being planetary-mass binaries if their ages are [<, ~]1 Gyr. Both WISE J1217+1626B and CFBDSIR J1458+1013B have strikingly blue Y - J colors compared to previously known T dwarfs, including their T9 primaries. Combining all available data, we find that Y - J color drops precipitously between the very latest T dwarfs and the Y dwarfs. The fact that this is seen in (coeval, mono-metallicity) binaries demonstrates that the color drop arises from a change in temperature, not surface gravity or metallicity variations among the field population. Thus, the T/Y transition established by near-IR spectra coincides with a significant change in the [approximate]1 mu m fluxes of ultracool photospheres. One explanation is the depletion of potassium, whose broad absorption wings dominate the far-red optical spectra of T dwarfs. This large color change suggests that far-red data may be valuable for classifying objects of [<, ~]500 K.