DISCOVERY OF A YOUNG L DWARF BINARY, SDSS J224953.47+004404.6AB*

摘要

We report discovery of a young 032 L dwarf binary, SDSS J2249+0044AB, found as the result of a Keck laser guide star adaptive optics imaging survey of young field brown dwarfs. Weak K I, Na I, and FeH features as well as strong VO absorption in the integrated-light J-band spectrum indicate a low surface gravity and hence young age for the system. From spatially resolved K-band spectra we determine spectral types of L3 ± 0.5 and L5 ± 1 for components A and B, respectively. SDSS J2249+0044A is spectrally very similar to G196–3B, an L3 companion to a young M2.5 field dwarf. Thus, we adopt 100?Myr (the age estimate of the G196–3 system) as the age of SDSS J2249+0044AB, but ages of 12-790?Myr are possible. By comparing our photometry to the absolute magnitudes of G196–3B, we estimate a distance to SDSS J2249+0044AB of 54 ± 16?pc and infer a projected separation of 17 ± 5 AU for the binary. Comparison of the luminosities to evolutionary models at an age of 100?Myr yields masses of 0.029 ± 0.006 and 0.022+0.006 –0.009 M ☉ for SDSS J2249+0044A and B, respectively. Over the possible ages of the system (12-790?Myr), the mass of SDSS J2249+0044A could range from 0.011 to 0.070 M ☉ and the mass of SDSS J2249+0044B could range from 0.009 to 0.065 M ☉. Evolutionary models predict that either component could be burning deuterium, which could result in a mass ratio as low as 0.4, or alternatively, a reversal in the luminosities of the binary. We find a likely proper motion companion, GSC 00568–01752, which lies 489 away (a projected separation of 2600?AU) and has Sloan Digital Sky Survey and Two Micron All Sky Survey colors consistent with an early M dwarf. We calculate a photometric distance to GSC 00568–01752 of 53 ± 15?pc, in good agreement with our distance estimate for SDSS J2249+0044AB. The space motion of SDSS J2249+0044AB shows no obvious coincidence with known young moving groups, though radial velocity and parallax measurements are necessary to refine our analysis. The unusually red near-IR colors, young age, and low masses of the binary make it an important template for studying planetary-mass objects found by direct imaging surveys.