Kinematics and history of the solar neighbourhood revisited

TL;DR

This study revisits the kinematics and star formation history of the solar neighborhood using new astrometric data, refining models of stellar motions and ages, and challenging previous assumptions about disc heating saturation.

Contribution

It provides updated measurements of local stellar kinematics and star formation history, and evaluates models of disc heating and star formation decline with new data and isochrones.

Findings

Velocity dispersions increase with stellar age, with different rates for vertical and horizontal components.

Star formation rate has been declining over time, with models favoring early peaks around 11-13 Gyr.

Disc heating saturation after 3 Gyr is unlikely; models suggest possible saturation after 4 Gyr.

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^\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.