  The hypothesis that strange quark matter is the true ground state of matter
has been investigated for almost four decades, but only a few works have
explored the dynamics of binary systems of quark stars. This is partly due to
the numerical challenges that need to be faced when modelling the large
discontinuities at the surface of these stars. We here present a novel
technique in which the EOS of a quark star is suitably rescaled to produce a
smooth change of the specific enthalpy across a very thin crust. The
introduction of the crust has been carefully tested by considering the
oscillation properties of isolated quark stars, showing that the response of
the simulated quark stars matches accurately the perturbative predictions.
Using this technique, we have carried out the first fully general-relativistic
simulations of the merger of quark-star binaries finding several important
differences between quark-star binaries and hadronic-star binaries with the
same mass and comparable tidal deformability. In particular, we find that
dynamical mass loss is significantly suppressed in quark-star binaries. In
addition, quark-star binaries have merger and post-merger frequencies that obey
the same quasi-universal relations derived from hadron stars if expressed in
terms of the tidal deformability, but not when expressed in terms of the
average stellar compactness. Hence, it may be difficult to distinguish the two
classes of stars if no information on the stellar radius is available. Finally,
differences are found in the distributions in velocity and entropy of the
ejected matter, for which quark-stars have much smaller tails. Whether these
differences in the ejected matter will leave an imprint in the electromagnetic
counterpart and nucleosynthetic yields remains unclear, calling for the
construction of an accurate model for the evaporation of the ejected quarks
into nucleons.
