Quark stars are challenging to confirm or exclude observationally because they can have similar masses and radii as neutron stars. By performing the first calculation of the nonequilibrium equation of state of decompressed quark matter at finite temperature, we determine the properties of the ejecta from binary quark-star or quark star–black hole mergers. We account for all relevant physical processes during the ejecta evolution, including quark nugget evaporation and cooling, and weak interactions. We find that these merger ejecta can differ significantly from those in neutron star mergers, depending on the binding energy of quark matter. For relatively high binding energies, quark star mergers are unlikely to produce $r$-process elements and kilonova signals. We propose that future observations of binary mergers and kilonovae could impose stringent constraints on the binding energy of quark matter and the existence of quark stars.

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• Nuclear astrophysics
• Quark matter
• Binary stars
• Neutron stars & pulsars