Resolved Spectroscopy of the T8.5 and Y0-0.5 Binary WISEPC J121756.91+162640.2AB

Abstract

We present 0.9 - 2.5 um resolved spectra for the ultracool binary WISEPC J121756.91+162640.2AB. The system consists of a pair of brown dwarfs that straddles the currently defined T/Y spectral type boundary. We use synthetic spectra generated by model atmospheres that include chloride and sulfide clouds (Morley et al.), the distance to the system (Dupuy & Kraus), and the radius of each component based on evolutionary models (Saumon & Marley) to determine a probable range of physical properties for the binary. The effective temperature of the T8.5 primary is 550 - 600 K, and that of the Y0 - Y0.5 secondary is 450 K. The atmospheres of both components are either free of clouds or have extremely thin cloud layers. We find that the masses of the primary and secondary are 30 and 22 M_Jup, respectively, and that the age of the system is 4 - 8 Gyr. This age is consistent with astrometric measurements (Dupuy & Kraus) that show that the system has kinematics intermediate between those of the thin and thick disks of the Galaxy. An older age is also consistent with an indication by the H - K colors that the system is slightly metal-poor.

21 pages which include 6 Figures and 3 Tables. Accepted on November 8 2013 for publication in ApJ

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• Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer
• The Y-Type Brown Dwarfs: Estimates of Mass and Age from New Astrometry, Homogenized Photometry and Near-Infrared Spectroscopy
• Near-infrared photometry of Y dwarfs: low ammonia abundance and the onset of water clouds
• Self-consistent Models of Y Dwarf Atmospheres with Water Clouds and Disequilibrium Chemistry
• Host star metallicity of directly imaged wide-orbit planets: implications for planet formation
• Discovery of a Low-Luminosity, Tight Substellar Binary at the T/Y Transition
• Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry
• 3.8um Imaging of 400-600K Brown Dwarfs and Orbital Constraints for WISEP J045853.90+643452.6AB
• Gamma-ray flux limits from brown dwarfs: Implications for dark matter annihilating into long-lived mediators
• TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with HPF