  The potential of tidal disruption of stars to probe otherwise quiescent
supermassive black holes cannot be exploited, if their dynamics is not fully
understood. So far, the observational appearance of these events has been
derived from analytical extrapolations of the debris dynamical properties just
after disruption. By means of hydrodynamical simulations, we investigate the
subsequent fallback of the stream of debris towards the black hole for stars
already bound to the black hole on eccentric orbits. We demonstrate that the
debris circularize due to relativistic apsidal precession which causes the
stream to self-cross. The circularization timescale varies between 1 and 10
times the period of the star, being shorter for more eccentric and/or deeper
encounters. This self-crossing leads to the formation of shocks that increase
the thermal energy of the debris. If this thermal energy is efficiently
radiated away, the debris settle in a narrow ring at the circularization radius
with shock-induced luminosities of $\sim 10-10^3 \, L_{\rm Edd}$. If instead
cooling is impeded, the debris form an extended torus located between the
circularization radius and the semi-major axis of the star with heating rates
$\sim 1-10^2 \, L_{\rm Edd}$. Extrapolating our results to parabolic orbits, we
infer that circularization would occur via the same mechanism in $\sim 1$
period of the most bound debris for deeply penetrating encounters to $\sim 10$
for grazing ones. We also anticipate the same effect of the cooling efficiency
on the structure of the disc with associated luminosities of $\sim 1-10 \,
L_{\rm Edd}$ and heating rates of $\sim 0.1-1 \, L_{\rm Edd}$. In the latter
case of inefficient cooling, we deduce a viscous timescale generally shorter
than the circularization timescale. This suggests an accretion rate through the
disc tracing the fallback rate, if viscosity starts acting promptly.
