First determination of the dynamical mass of a binary L dwarf

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Abstract
We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover ~36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9 days. The corresponding total mass is 0.146 with uncertainties depending on the distance. Spatially resolved low resolution optical (550–1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components ( for the primary and for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and K S bands) and Keck I (K S band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H–R diagram. The binary system is most likely formed by a primary with a mass of and a secondary with a mass of , thus clearly substellar, for an age of approximately Myr. Hα variability indicates chromospheric and/or magnetic activity.
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