  We have evolved mergers of equal-mass binary quark stars, the total mass of
which is close to the mass shedding limit of uniformly rotating configurations,
in fully general relativistic hydrodynamic simulations, aimed at investigating
the post-merger outcomes. In particular, we have identified the threshold mass
for prompt black hole formation after the merger, by tracing the minimum lapse
function as well as the amount of ejected material during the merger
simulation. A semi-analytical investigation based on the angular momentum
contained in the merger remnant is also performed to verify the results. For
the equation of state considered in this work, the maximum mass of TOV
solutions for which is 2.10 $M_\odot$, the threshold mass is found between 3.05
and 3.10 $M_\odot$. This result is consistent (with a quantitative error
smaller than 1%) with the universal relation derived from the numerical results
of symmetric binary neutron star mergers. Contrary to the neutron star case,
the threshold mass is close to the mass shedding limit of uniformly rotating
quark star. Consequently, we have found that binary quark stars with total mass
corresponding to the long-lived supramassive remnant for neutron star case,
could experience collapse to black hole within several times dynamical
timescale, making quark stars as exceptions of the commonly accepted
post-merger scenarios for binary neutron star mergers. We have suggested
explanation for both the similarity and the difference, between quark stars and
neutron stars.
