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 ~43 arcsec (480 au physical projected separation) north-west of its primary -the exoplanet host star HD 3651 at 11 pc. The companion, HD 3651 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. HD 3651 B is not detected in the POSS-II B-, R- or I-band images, indicating that this object is fainter than ~20 mag in the B- and R-band and fainter than ~ 19 mag in the I-band. With the Hipparcos distance of HD 3651 of 11 pc, the absolute magnitude of HD 3651 B 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 HD 3651 B 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 HD 3651 B 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).