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

We present the discovery of seven new T dwarfs identified in the Two MicronAll Sky Survey. Low-resolution (R$\sim$150) 0.8--2.5 micron spectroscopyobtained with the IRTF SpeX instrument reveal the characteristic H$_2$O andCH$_4$ bands in the spectra of these brown dwarfs. Comparison to spectralstandards observed with the same instrument enable us to derive classificationsof 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 linestrengths consistent with previously observed trends with spectral type.Follow-up imaging observations provide proper motion measurements for thesesources, 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 ofthe Sun. This source also exhibits a depressed K-band peak reminiscent of thepeculiar T dwarf 2MASS 0937+2931, and may be a metal-poor or old, high-massbrown dwarf. We also present low resolution SpeX data for a set of M and L-typedwarf, subdwarf, and giant comparison stars used to classify 59 additionalcandidates identified as background stars. These are primarily M5-M8.5 dwarfs,many exhibiting H I Paschen $\gamma$, but include three candidate ultracool Msubdwarfs and one possible early-type L subdwarf.Comment: 27 pages incl. 9 figures and 8 tables, accepted for publication AJ, May 2004; note that findercharts are provided separately as jpg file