Cool Stars 14 - Submitted Abstract # 46
This version created on 05 October 2006

HD3651B:  the first directly imaged brown dwarf companion of an
exoplanet host star


M. Mugrauer
A. Seifahrt
R. Neuhaeuser
T. Mazeh

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.  The companion, HD3651B, clearly shares
the proper motion of the exoplanet host star in our four images,
obtained with ESO/NTT and UKIRT, 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- and 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 60MJup
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).

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