Low-metallicity L and T Dwarfs J. Davy Kirkpatrick (IPAC/Caltech) Dagny L. Looper (IfA/UHawaii) Adam J. Burgasser (MIT) Travis R. Barman (Lowell Obs.)

The 2MASS Proper Motion Survey 2MASS includes multiple-epoch coverage of ~4700 sq. deg. We have used these data to look for near-IR proper motion objects. We have uncovered ~150 motion objects with no optical counterparts in B- or R- band DSS data. We have followed up most of these using SpeX at the IRTF. The goal of this survey is to uncover halo (low-metallicity) brown dwarfs whose colors are poorly known, along with other cool, nearby exotica. I-band DSS image from 1989 (blue) and J-band 2MASS image from 1999 (red). Motion object is marked. Combined image showing motion of source over 10 years.

Example subdwarfs from our survey SpeX prism spectra of two of our subdwarf discoveries (red) compared to standard L dwarfs of the same spectral type (black). The most noticeable difference is the suppressed H- and K-bands in the subdwarfs. This is due to the dominance of collision-induced absorption by H2 in these low- metallicity atmospheres.

Why are subdwarf brown dwarfs important? Star Formation: These discoveries show further evidence that low- metallicity clouds still produce very low-mass objects. Although more of these very cool subdwarfs are needed before that formation efficiency can be compared to brown dwarf formation at current epochs, this survey has nonetheless provided additional, very important clues on searching for other such objects using photometry alone. Extrasolar Giant Planets: These subdwarfs allow us to study cool atmospheres with low values of [M/H], which is an important test of theoretical models. These objects have temperatures comparable to some of the newly found extrasolar planets and thus provide low-metallicity fiducials for the study of gas giant atmospheres.