Michael C. Liu: Methane Dwarf from IfA-Deep Survey

Discovery of a Methane Dwarf from the IfA-Deep Survey

Michael C. Liu, Richard Wainscoat, Eduardo L. Martin, Brian Barris, John Tonry (IfA/Hawaii)

ApJL, 2002, 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 micron) 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 heightof ultracool (L and T) dwarfs.

Results

Optical and near-IR imaging of IfA 0230-Z1. It's a boomer! Each image is 30" on a side. The I and z'-band imaging is from the mighty Suprime-Cam on the Subaru Telescope, slightly trailed because of a telescope guiding problem. (Even the mighty have their off days.) The J-band image is from the SpeX spectrograph on IRTF.

Keck/NIRSPEC spectrum of IfA 0230-Z1 compared with spectra of local T dwarfs from Geballe et al (2002). The blue depression of the continuum is from water absorption. Methane absorption is present, but modest compared to most known T dwarfs. The inferred spectral type is T3-T4, with a resulting distance of ~45 pc. See our paper for details.

The Paper: available from astro-ph.