New nearby, bright southern ultracool dwarfs

Abstract

We report the discovery of 21 hitherto unknown bright southern ultracool dwarfs with spectral types in the range M7 to L5.5, together with new observations of a further three late M dwarfs previously confirmed. Three more objects are already identified in the literature as high proper motion stars; we derive their spectral types for the first time. All objects were selected from the Two Micron All-Sky Survey (2MASS) all-sky and SuperCOSMOS point source data bases on the basis of their optical/near-infrared colours, J-band magnitudes and proper motions. Low-resolution (R∼ 1000)JH spectroscopy with the European Southern Observatory (ESO)/New Technology Telescope (NTT) Son of Isaac (SOFI) spectrograph has confirmed the ultracool nature of 24 targets, out of a total of 25 candidates observed. Spectral types are derived by direct comparison with template objects and compared to results from H2O and FeH indices. We also report the discovery of one binary, as revealed by SOFI acquisition imaging; spectra were taken for both components. The spectral types of the two components are L2 and L4 and the distance ∼19 pc. Spectroscopic distances and transverse velocities are derived for the sample. Two ∼L5 objects lie only at ∼0 pc distant. Such nearby objects are excellent targets for further study to derive their parallaxes and to search for fainter, later companions with adaptive optics and/or methane imaging.