Kinematics and age of stellar populations in the solar neighbourhood from Hipparcos data

Abstract

The large sample of main sequence stars with Hipparcos parallaxes and estimates of the radial velocity has been used to study the relations among kinematics, age and heavy element content for the solar neighbourhood. The samples with V> -30 km $\rm s^{-1}$, although including stars as old as 10^10 yr, have a very young component (age $\simgt$ 10^7 yr), and have a metallicity close to the solar one. Most of the stars with space velocity component V < -40 km $\rm s^{-1}$ have a minimum age of about 2 10^9 yr. Stars with V \simlt -80 km $\rm s^{-1}$ share a common age \simgt $10^{10}$ yr. The distribution in \Feh of these latter objects suggests a decrease in metal content with increasing $| V |$ till \Feh < -2. For V < -180 km $\rm s^{-1}$ only the metal poor component ([Fe/H] < -0.7) is found. The common age for large space velocity suggests that no substantial age spread exists in the inner halo, at least in the local sample.