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 2MASS proper-motion search, which uses multiepoch 2MASS observations covering ∼4700 deg² of sky. 2MASS J18212815+1414010 and 2MASS J21481628+4003593 were overlooked by earlier surveys due to their faint optical magnitudes and their proximity to the Galactic plane (10° ≤ |b| ≤ 15°). Assuming that both dwarfs are single, we derive spectrophotometric distances of ∼10 pc, thus increasing the number of known L dwarfs within 10 pc to 10. In the near-infrared, 2MASS J21481628+4003593 shows a triangular H-band spectrum, strong CO absorption, and a markedly red J - K_s color (2.38 ± 0.06) for its L6 optical spectral type. 2MASS J18212815+1414010 also shows a triangular H-band spectrum and a slightly red J - K_s 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 for both of these objects. We examine several scenarios to explain the underlying cause for their enhanced dust content and find that a metal-rich atmosphere or a low surface gravity are consistent with these results. 2MASS J18212815+1414010 may be young (and therefore have a low surface gravity) based on its low tangential velocity of 10 km s^_1. On the other hand, 2MASS J21481628+4003593 has a high tangential velocity of 62 km s^_1 and is therefore likely old. Hence, high metallicity and low surface gravity may lead to similar effects.