Kinematics and history of the solar neighbourhood revisited
  We use proper motions and parallaxes from the new reduction of Hipparcos data
and Geneva-Copenhagen radial velocities for a complete sample of ~15000
main-sequence and subgiant stars, and new Padova isochrones to constrain the
kinematics and star-formation history of the solar neighbourhood. We rederive
the solar motion and the structure of the local velocity ellipsoids. When the
principal velocity dispersions are assumed to increase with time as t^\beta,
the index \beta is larger for \sigma_W (\beta_W~0.45) than for \sigma_U
(\beta_U~0.31). For the three-dimensional velocity dispersion we obtain
\beta=0.35. We exclude saturation of disc heating after ~3 Gyr as proposed by
Quillen & Garnett(2000). Saturation after >~4 Gyr combined with an abrupt
increase in velocity dispersion for the oldest stars cannot be excluded. For
all our models the star-formation rate is declining, being a factor 2-7 lower
now than it was at the beginning. Models in which the SFR declines
exponentially favour very high disc ages between 11.5 and 13 Gyr and exclude
ages below ~10.5 Gyr as they yield worse fits to the number density and
velocity dispersion of red stars. Models in which the SFR is the sum of two
declining exponentials representing the thin and thick discs favour ages
between 10.5 and 12 Gyr with a lower limit of ~10.0 Gyr. Although in our models
the star-formation rate peaked surprisingly early, the mean formation time of
solar-neighbourhood stars is later than in ab-initio models of galaxy
formation, probably on account of weaknesses in such models.
