Using Hipparcos parallaxes and proper
motions together with radial velocity data and
individual ages estimated from isochones, the
velocity ellipsoid has been determined as a
function of age. On the basis of the available
kinematic data two different samples were
considered: a first one (7789 stars) for which
only tangential velocities were calculated and a
second one containing 3104 stars with available U,
V and W velocity components and total velocities ≤
65 km.s-1.
The main conclusions are: -Mixing is not
complete at about 0.8-1 Gyr. -The shape of the
velocity ellipsoid changes with time getting
rounder from σu/σv/σ-w = 1/0.63/0.42 ± 0.04 at
about 1 Gyr to1/0.7/0.62 ±0.04 at 4-5 Gyr. -The
age-velocity-dispersion relation (from the sample
with kinematical selection) rises to a maximum,
thereafter remaining roughly constant; there is no
dynamically significant evolution of the disk
after about 4-5 Gyr. -Among the stars with solar
metallicities and log(age) > 9.8 two groups are
identified: one has typical thin disk
characteristics, the other is older than 10 Gyr
and lags the LSR at about 40
km.s-1 . -The variation of
the tangential velocity with age(without selection
on the tangential velocity) shows a discontinuity
at about 10 Gyr, which may be attributed to stars
typically of the thick disk populations for ages
> 10 Gyr.
Accessing temperature waves: A dispersion relation perspective
