Kinematics and history of the solar neighbourhood revisited

Abstract

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^β, the index β is larger for σ_W (β_W ~ 0.45) than for σ_U (β_U ~ 0.31). For the three-dimensional velocity dispersion, we obtain β = 0.35. We exclude saturation of disc heating after ~3Gyr as proposed by Quillen & Garnett. Saturation after >~4Gyr 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 13Gyr and exclude ages below ~10.5Gyr 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 12Gyr with a lower limit of ~10.0Gyr. 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.