Discovery of a Methane Dwarf from the I[CLC]f[/CLC]A Deep Survey

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Abstract
We present the discovery of a distant methane dwarf, the first from the Institute for Astronomy (IfA) Deep Survey. The object ("IfA 0230-Z1") was identified from deep optical I- and z'-band imaging, being conducted as an IfA-wide collaboration using the prime-focus imager Suprime-Cam on the Subaru 8.2 m Telescope. IfA 0230-Z1 is extremely red in the Iz'J (0.8-1.2 μm) bands but relatively blue in J-H; such colors are uniquely characteristic of T dwarfs. A near-IR spectrum taken with the Keck Telescope shows strong H2O absorption and a continuum break indicative of CH4, confirming that the object has a very cool atmosphere. Comparison with nearby T dwarfs gives a spectral type of T3-T4 and a distance of ~45 pc. Simple estimates based on previous T dwarf discoveries suggest that the IfA survey will find a comparable number of T dwarfs as the 2 Micron All Sky Survey, albeit at a much larger average distance. We also discuss the survey's ability to probe the Galactic scale height of ultracool (L and T) dwarfs.
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