The 2MASS Wide-Field T Dwarf Search. III. Seven New T Dwarfs and Other Cool Dwarf Discoveries

Abstract

We present the discovery of seven new T dwarfs identified in the Two Micron All Sky Survey. Low-resolution (R$sim$150) 0.8--2.5 micron spectroscopy obtained with the IRTF SpeX instrument reveal the characteristic H$_2$O and CH$_4$ bands in the spectra of these brown dwarfs. Comparison to spectral standards observed with the same instrument enable us to derive classifications of T3 to T7 for the objects in this sample. Moderate-resolution (R$sim$1200) near-infrared spectroscopy for a subset of these discoveries reveal K I line strengths consistent with previously observed trends with spectral type. Follow-up imaging observations provide proper motion measurements for these sources, ranging from $<$ 0$farcs$1 to 1$farcs$55 yr$^{-1}$. One object, 2MASS 0034+0523, has a spectrophotometric distance placing it within 10 pc of the Sun. This source also exhibits a depressed K-band peak reminiscent of the peculiar T dwarf 2MASS 0937+2931, and may be a metal-poor or old, high-mass brown dwarf. We also present low resolution SpeX data for a set of M and L-type dwarf, subdwarf, and giant comparison stars used to classify 59 additional candidates identified as background stars. These are primarily M5-M8.5 dwarfs, many exhibiting H I Paschen $gamma$, but include three candidate ultracool M subdwarfs and one possible early-type L subdwarf.