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 similar to 15 000 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) approximate to 0.45) than for Sigma(U) (beta(U) approximate to 0.31). For the three-dimensional velocity dispersion, we obtain beta = 0.35. We exclude saturation of disc heating after similar to 3 Gyr as proposed by Quillen & Garnett. Saturation after greater than or similar to 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 (SFR) is declining, being a factor of 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 similar to 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 similar to 10.0 Gyr. Although in our models the SFR 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.