  Stars in the Galactic disc, including the Solar system, have deviated from
their birth orbits and have experienced radial mixing and vertical heating. By
performing hydrodynamical simulations of a galactic disc, we investigate how
much tracer particles, which are initially located in the disc to mimic newborn
stars and the thin and thick disc stars, are displaced from initial
near-circular orbits by gravitational interactions with giant molecular clouds
(GMCs). To exclude the influence of other perturbers that can change the
stellar orbits, such as spiral arms and the bar, we use an axisymmetric form
for the entire galactic potential. First, we investigate the time evolution of
the radial and vertical velocity dispersion $\sigma_R$ and $\sigma_z$ by
comparing them with a power law relation of $\sigma \propto t^{\beta}$.
Although the exponents $\beta$ decrease with time, they keep large values of
0.3 $\sim$ 0.6 for 1 Gyr, indicating fast and efficient disc heating. Next, we
find that the efficient stellar scattering by GMCs also causes a change in
angular momentum for each star and, therefore, radial migration. This effect is
more pronounced in newborn stars than old disc stars; nearly 30 per cent of
stars initially located on the galactic mid-plane move more than 1 kpc in the
radial direction for 1 Gyr. The dynamical heating and radial migration
drastically occur in the first several hundred Myr. As the amplitude of the
vertical oscillation increases, the time spent in the galactic plane, where
most GMCs are distributed, decreases, and the rate of an increase in the
heating and migration slows down.
