HD 3651 B:: the first directly imaged brown dwarf companion of an exoplanet host star

In the course of our ongoing multiplicity study of exoplanet host stars we detected a faint companion located at similar to 43 arcsec (480 au physical projected separation) north-west of its primary the exoplanet host star HD3651 at 11 pc. The companion, HD3651 B, clearly shares the proper motion of the exoplanet host star in our four images, obtained with the European Southern Observatory New Technology Telescope and the United Kingdom Infrared Telescope, spanning three years in epoch difference. The magnitude of the companion is H = 16.75 +/- 0.16 mag, the faintest co-moving companion of an exoplanet host star imaged directly. HD3651B is not detected in the POSS-II B-, R- or I-band images, indicating that this object is fainter than similar to 20 mag in the B- and R-band and fainter than similar to 19 mag in the I-band. With the Hipparcos distance of HD3651 of 11 pc, the absolute magnitude of HD3651B is about 16.5 mag in the H band. Our H-band photometry and the Baraffe et al. (2003) evolutionary models yield a mass of HD3651B to be 20 to 60 M-Jup (Jupiter masses) for assumed ages between 1 and 10 Gyr. The effective temperature ranges between 800 and 900 K, consistent with a spectral type of T7 to T8. We conclude that HD3651B is a brown dwarf companion, the first of its kind directly imaged as a companion of an exoplanet host star, and one of the faintest T dwarfs found in the solar vicinity (within 11 pc).