Discovery of 11 New T Dwarfs in the Two Micron All Sky Survey, Including a Possible L/T Transition Binary

Abstract

We present the discovery of 11 new T dwarfs, found during the course of a photometric survey for mid-to-late T dwarfs in the 2MASS Point Source Catalog and from a proper-motion-selected sample of ultracool dwarfs in the 2MASS Working Database. Using the NASA Infrared Telescope Facility SpeX spectrograph, we obtained low-resolution (R ~ 150) spectroscopy, allowing us to derive near-infrared spectral types of T2-T8. We also present improved signal-to-noise ratio SpeX low-resolution spectroscopy of the near-infrared T0 standard SDSS J120747.17+024424.8 and T1 standard SDSSp J083717.22-000018.3. One of these new T dwarfs, 2MASS J13243559+6358284, was also discovered independently by Metchev et al. This object is spectroscopically peculiar and possibly a binary and/or very young (<300 Myr). We specifically attempted to model the spectrum of this source as a composite binary to reproduce its peculiar spectral characteristics. The latest type object in our sample is a T8 dwarf, 2MASS J07290002-3954043, now one of the four latest type T dwarfs known. All 11 T dwarfs are nearby given their spectrophotometric distance estimates, with one T dwarf within 10 pc and eight additional T dwarfs within 25 pc, if single. These new additions increase the 25 pc census of T dwarfs by ~14%. Their proximity offers an excellent opportunity to probe for companions at separations closer than are possible for more distant T dwarfs.