AAS Meeting #193 - Austin, Texas, January 1999
Session 98. Low Mass Stars and Brown Dwarfs
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[98.03] Progress Report on the Search for the Elusive Methane (T-type) Field Brown Dwarf

A. J. Burgasser, J. D. Kirkpatrick, N. Reid (Caltech/IPAC), J. E. Gizis, M. F. Skrutskie (UMASS), D. G. Monet, H. Harris, C. C. Dahn (USNO, Flagstaff), J. W. Liebert (U. Arizona)

We present preliminary results from our search for methane (T-type) brown dwarfs in the field. Up to now, the only objects cool enough to show methane spectral features in the infrared (Teff \lesssim 1200\circK) have been one stellar companion (Gl 229B) and objects in our own solar system (e.g. Jupiter, Saturn). By identifying sources from the 2MASS point source catalog that have large optical-infrared colors (R-Ks > 7.0) and blue infrared colors (J-Ks < 1.0; note Gl 229B J-Ks = -0.1), we hope to identify similar objects in the solar neighborhood, thus extending the current L dwarf sequence into cooler ``T-type'' regimes (current L8 \approx 1400\circK).

The most recent search on ~ 1500 square degrees of Northern Hemisphere sky yielded 264 candidate objects with J < 16, Ks < 15, no optical counterpart (effective R-Ks \gtrsim 6), and no correlation with known minor planets. These objects were subsequently observed on the Palomar 60'' Infrared Camera (IRC) in October 1998 for photometric confirmation (to eliminate proper motion stars, uncatalogued minor planets and imaging artifacts. Of 166 candidates examined, 43 were not confirmed (most presumably being uncatalogued minor planets), 22 had optical counterparts or proper motion companions, and 101 were confirmed infrared sources with no obvious optical counterpart (R \gtrsim 22 for POSS-II). Of these, eight had 1.0 \geq J-Ks > 0.9, eight had 0.9 \geq J-Ks > 0.8, and six had J-Ks \leq 0.8. No objects were found to have J-Ks \leq 0.0 like Gl 229B, although one object was confirmed with J-Ks = -0.46 and an optical counterpart. In all, 32% of objects identified from the 2MASS point source survey with J-Ks \leq 1.0 were confirmed as real infrared sources.

In the upcoming months, we will be obtaining optical photometry of our confirmed infrared sources at the Palomar 60'' to derive precise R-Ks colors; from these measurements, objects with the largest R-Ks (R-Ks = 10.8 for Gl 229B) and bluest J-Ks will be spectroscopically observed in optical and near-infrared bands at Palomar 200'', Keck, and UKIRT in order to establish their stellar type. We hope that from this list the first field methane brown dwarf will be identified.

The author(s) of this abstract have provided an email address for comments about the abstract: diver@cco.caltech.edu

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