  We study the stars of the binary system 16 Cygni to determine with high
precision their chemical composition. Knowing that the component B has a
detected planet of at least 1.5 Jupiter masses, we investigate if there are
chemical peculiarities that could be attributed to planet formation around this
star. We perform a differential abundance analysis using high resolution (R =
81,000) and high S/N (~700) CFHT/ESPaDOnS spectra of the 16 Cygni stars and the
Sun; the latter was obtained from light reflected of asteroids. We determine
differential abundances of the binary components relative to the Sun and
between components A and B as well. We achieve a precision of about 0.005 dex
and a total error ~0.01 dex for most elements. The effective temperatures and
surface gravities found for 16 Cyg A and B are Teff = 5830+/-7 K, log g =
4.30+/-0.02 dex, and Teff = 5751+/-6 K, log g = 4.35+/-0.02 dex, respectively.
The component 16 Cyg A has a metallicity ([Fe/H]) higher by 0.047+/-0.005 dex
than 16 Cyg B, as well as a microturbulence velocity higher by 0.08 km/s. All
elements show abundance differences between the binary components, but while
the volatile difference is about 0.03 dex, the refractories differ by more and
show a trend with condensation temperature, which could be interpreted as the
signature of the rocky accretion core of the giant planet 16 Cyg Bb. We
estimate a mass of about 1.5-6 M_Earth for this rocky core, in good agreement
with estimates of Jupiter's core.
