We present a computation of bulk viscosity arising from hyperonic processes in matter at densities and temperatures typical of neutron star mergers. To deal with the high temperatures in this environment we go beyond the Fermi surface approximation in our rate calculations and numerically evaluate the full phase space integral. We include processes where quarks move between baryons via meson exchange: these have been largely omitted in previous analyses but provide the dominant contribution to the bulk viscosity. We obtain the dissipation times for harmonic oscillations at the frequencies seen in merger simulations, and show that hyperon bulk viscosity can be highly relevant at densities just below the onset of the first hyperon species and temperatures up to T = 5 MeV, with dissipation times as fast as 9 ms.

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