  First results of numerical simulations are presented which compute the
dynamical evolution of a neutron star with a mass slightly below the minimum
stable mass by means of a new implicit (general relativistic) hydrodynamic
code. We show that such a star first undergoes a phase of quasi-static
expansion, caused by slow nuclear $\beta$-decays, lasting for about 20 seconds,
but then explodes violently. The kinetic energy of the explosion is around
$10^{49} erg$, the peak luminosity in electron anti-neutrinos is of order
$10^{52} erg/s$, and the thermodynamic conditions of the expanding matter are
favorable for r-process nucleosynthesis. These results are obtained for the
Harrison-Wheeler equation of state and a simple and, possibly, unrealistic
treatment of $\beta$-decay rates and nuclear fission, which were adopted for
comparison with previous works. However, we do not expect that the outcome will
change qualitatively if more recent nuclear input physics used. Although our
study does not rely on a specific scenario ofhow a neutron star starting from a
bigger (and stable) masscan reach the dynamical phase, we assume that the final
mass-loss event happens on a very short time scale, i.e., on a time scale
shorter than a sound-crossing time, by removing a certain amount of mass as an
initial perturbation. This assumption implies that the star has no time to
adjust its nuclear composition to the new mass mass through a sequence of
quasi-equilibria. In the latter case, however, there exists no stable
configuration below the minimum mass, because the equation of state of fully
catalyzed matter is too soft. Therefore, the dynamics of the explosion will not
be too different from what have obtained if different initial perturbations are
assumed.
