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The very nearby M/T dwarf binary SCR 1845-6357
Context: The recently discovered star SCR 1845-6357 (hereafter SCR 1845)is the first late M/T dwarf binary discovered. SCR 1845 is a particularobject due to its tight orbit (currently around 4 AU) and its proximityto the Sun (3.85 pc). Aims: We present spatially resolvedVLT/NACO images and low resolution spectra of SCR 1845 in the J, H and Knear-infrared bands. Since the T dwarf companion, SCR 1845B, is so closeto the primary SCR 1845A, orbital motion is evident even within a year.Following the orbital motion, the binary's mass can be measuredaccurately within a decade, making SCR 1845B a key T-dwarfmass-luminosity calibrator. The NIR spectra allow for accuratedetermination of spectral type and also for rough estimates of theobject's physical parameters. Methods: The spectral type of SCR1845B is determined by direct comparison of the flux calibrated JHKspectra with T dwarf standard template spectra and also by NIR spectralindices obtained from synthetic photometry. Constrained values forsurface gravity, effective temperature and metallicity are derived bycomparison with model spectra. Results: Our data prove that SCR1845B is a brown dwarf of spectral type T6 that is co-moving with andtherefore gravitationally bound to the M8.5 primary. Fitting the NIRspectrum of SCR 1845B to model spectra yields an effective temperatureof about 950 K and a surface gravity log{g}=5.1 (cgs) assuming solarmetallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupitermasses and 1.8 to 3.1 Gyr.Based on observations collected at the European Southern Observatory,Chile, through proposal 276.C-5038(A).
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