The evolution of the Milky Way monitored in the solar

In this review we concentrate on the dynamical evolution of the Milky Way as monitored in the solar neighbourhood. The relevant data sets are presented and discussed in detail. In the second part we review various mechanisms, which drive the dynamical evolution. 1. Dynamical evolution of the galactic disk traced in the solar neighbourhood The key data set, which we use for this study, is the Fourth Catalogue of Nearby Stars (hereafter referred to as CNS4), which has been now completed in its preliminary form (cf. Jahreiß et al. 1998, 1999). The catalogue represents the most complete inventory of stars within a distance of 25 pc from the Sun. It contains data of about 3000 stars, most of them improved by Hipparcos (ESA 1997) parallaxes. For instance, the parallaxes of 1411 stars out of the 1761 stars with parallaxes more accurate than 10% are Hipparcos measurements. Statistical tests have shown that the CNS4 is complete for stars with absolute magnitudes M V ≤ 8 m , which includes the K stars. For M stars the CNS4 becomes increasingly incomplete and kinematically biased towards high proper motions. Fig. 1 illustrates the kinematical evolution of the galactic disk, where the total velocity dispersions of the peculiar velocities of the stars, i.e. the quadrat-ical sum of the dispersions of the three velocity components, are plotted versus the ages of the stars. For this purpose we have extracted three data sets from the CNS4. The first set includes all main sequence stars down to late type K stars with known space velocities. These have been grouped according to B − V colour and the age of each group is assumed to be one half of the average main sequence life time of the stars, or for long lived late type stars one half of the age of the galactic disk, for which we have adopted a value of 10 Gyrs (cf. also Jahreiß et al. 1998, 1999). The second set of stars is given by the McCormick K and M dwarfs with known space velocities. The McCormick stars have been detected by an objective prism survey (Vyssotsky 1963) and are thus free of kinematical bias. Their ages can be determined using the emission intensities of the emission kernels of the Calcium H and K absorption lines, because the emission intensity decreases with increasing age of a star. Emission intensities have been indexed by Wilson …