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Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich?
We present the discovery of two nearby L dwarfs from our 2MASSproper-motion search, which uses multiepoch 2MASS observations covering~4700 deg2 of sky. 2MASS J18212815+1414010 and 2MASSJ21481628+4003593 were overlooked by earlier surveys due to their faintoptical magnitudes and their proximity to the Galactic plane(10deg<=|b|<=15°). Assuming that both dwarfs aresingle, we derive spectrophotometric distances of ~10 pc, thusincreasing the number of known L dwarfs within 10 pc to 10. In thenear-infrared, 2MASS J21481628+4003593 shows a triangular H-bandspectrum, strong CO absorption, and a markedly red J-Ks color(2.38+/-0.06) for its L6 optical spectral type. 2MASS J18212815+1414010also shows a triangular H-band spectrum and a slightly redJ-Ks color (1.78+/-0.05) for its L4.5 optical spectral type.Both objects show strong silicate absorption at 9-11 μm.Cumulatively, these features imply an unusually dusty photosphere forboth of these objects. We examine several scenarios to explain theunderlying cause for their enhanced dust content and find that ametal-rich atmosphere or a low surface gravity are consistent with theseresults. 2MASS J18212815+1414010 may be young (and therefore have a lowsurface gravity) based on its low tangential velocity of 10 kms-1. On the other hand, 2MASS J21481628+4003593 has a hightangential velocity of 62 km s-1 and is therefore likely old.Hence, high metallicity and low surface gravity may lead to similareffects.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan. Some of the datapresented herein were obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
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