Discovery of the Closest Brown Dwarf to the Sun? a Methane Rich Brown Dwarf around the Nearby

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered (Hambly et al. 2004) M8.5 star just 3.5 pc from the sun (Deacon et al. 2005). Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4 AU (1.170”±0.003” on the sky, PA=170.20◦±0.13◦) and a ∆F1(1.575 μm) of 3.57±0.057 mag. This substellar companion has an H magnitude of 13.3 −0.21, and an absolute H magnitude of 15.5 −0.20. The unique Simultaneous Differential Imager (SDI) consists of 3 narrowband filters placed around the 1.6 μm methane absorption feature characteristic of T-dwarfs (Teff < 1200 K). The flux of the substellar companion drops by a factor of 2.7±0.1 between the SDI F1(1.575 μm) filter and the SDI F3(1.625 μm) filter, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T6±1 for the companion based on the strength of the break. The chances that this object is a background T dwarf are vanishing small – and there is no isolated background Tdwarf anywhere near these coordinates in the field according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845-6357B. For an age range of 100 Myr 10 Gyr and spectral type range of T5-T7, we find a mass range of 8.5 60 MJup for SCR 1845B from the Baraffe et al. (2003) COND models. This is likely the closest brown dwarf to the Sun (in the 16th closest stellar system to the sun.) Subject headings: instrumentation: adaptive optics — binaries: brown dwarfs