  We are carrying out a search for planets around a sample of solar twin stars
using the HARPS spectrograph. The goal of this project is to exploit the
advantage offered by solar twins to obtain chemical abundances of unmatched
precision. This survey will enable new studies of the stellar composition --
planet connection. Here we used the MIKE spectrograph on the Magellan Clay
Telescope to acquire high resolution, high signal-to-noise ratio spectra of our
sample stars. We measured the equivalent widths of iron lines and used strict
differential excitation/ionization balance analysis to determine atmospheric
parameters of unprecedented internal precision (DTeff=7K, Dlogg=0.019,
D[Fe/H]=0.006dex, Dvt=0.016km/s). Reliable relative ages and highly precise
masses were then estimated using theoretical isochrones. The spectroscopic
parameters we derived are in good agreement with those measured using other
independent techniques. The root-mean-square scatter of the differences seen is
fully compatible with the observational errors, demonstrating, as assumed thus
far, that systematic uncertainties in the stellar parameters are negligible in
the study of solar twins. We find a tight activity-age relation for our sample
stars, which validates the internal precision of our dating method.
Furthermore, we find that the solar cycle is perfectly consistent both with
this trend and its star-to-star scatter. We present the largest sample of solar
twins analyzed homogeneously using high quality spectra. The fundamental
parameters derived from this work will be employed in subsequent work that aims
to explore the connections between planet formation and stellar chemical
composition.
