2006MNRAS.373L..31M

In the course of our ongoing multiplicity study of exoplanet host stars we detected a faint companion located at ∼43arcsec (480au physical projected separation) north-west of its primary - the exoplanet host star HD3651 at 11pc. The companion, HD3651B, 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±0.16mag, 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 ∼20mag in the B- and R-band and fainter than ∼19mag in the I-band. With the Hipparcos distance of HD3651 of 11pc, the absolute magnitude of HD3651B is about 16.5mag 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 60M_Jup(Jupiter masses) for assumed ages between 1 and 10Gyr. The effective temperature ranges between 800 and 900K, 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 11pc).