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Abstract: We have monitored the benchmark L dwarf binary Kelu-1AB over the past 3 yearsto derive improved spectral types and luminosities for the individualcomponents. The astrometric measurements enable us to compute the orbitalparameters and result in the first dynamical mass estimate for the system. Weobtained resolved high angular resolution, near-IR images with HST and theVLT-NACO adaptive optics instrument in the J, H and Ks bands. In addition weused NACO to achieve the first spatially resolved mid-resolution spectra in theH- and K-band for a precise spectral type determination. The near-IR spectrumof Kelu-1A reveals a distinct dip in the H-band providing evidence that Kelu-1A itself is a spectroscopic binary. We derive spectral types of L0.5+- 0.5 forKelu- 1 Aa and T7.5+-1 for Kelu-1 Ab. Kelu-1B is classified as spectral typeL3pec +-1.5. From the relative orbit, we derive an orbital period of 38 +8 -6years and a semi-major axis of 6.4+2.4 -1.3 AU. This yields the firstdynamically determined total system mass of 177 +113 -55 Mjup for the Kelu-1ABsystem, with the uncertainties mainly attributed to the error of the systemdistance measurement and the yet missing apastron passage. The derivedproperties of Kelu-1AB allow us to test common theoretical models. Thecomparison of our results with color-magnitude diagrams based on evolutionarymodels yields a slightly revised age estimate 0.3 - 0.5 Gyr and a discrepancybetween dynamically and theoretically derived masses, stressing the importancefor further dynamical mass determinations of brown dwarf binaries.

Autor: M.B. Stumpf, W. Brandner, Th. Henning, H. Bouy, R. Koehler, F. Hormuth, V. Joergens, M. Kasper


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