Discovery of a Methane Dwarf from the IfA-Deep Survey.pdfVIP

a r X i v : a s t r o - p h / 0 2 0 2 5 0 8 v 1 2 8 F e b 2 0 0 2 Astrophysical Journal Letters, in press Preprint typeset using LATEX style emulateapj v. 14/09/00 DISCOVERY OF A METHANE DWARF FROM THE IFA-DEEP SURVEY1 Michael C. Liu2, Richard Wainscoat, Eduardo L. Mart?in, Brian Barris, John Tonry Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822 mliu@ifa.hawaii.edu Astrophysical Journal Letters, in press 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 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 2MASS 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. Subject headings: stars: low-mass, brown dwarfs — infrared: stars — surveys 1. introduction In the past few years, observational studies of sub- stellar objects have been revolutionized, in large part from the advent of wide-field sky surveys, namely the 2- Micron All-Sky Survey (2MASS; Skrutskie et al. 1997), the Deep Near-Infrared Survey of the Southern Sky (DE- NIS; Epchtein et al. 1999), and the Sloan Digital Sky Sur- vey (SDSS; York et al. 2000). The coolest of the known substellar objects, the T dwarfs, are identified by