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Short gamma-ray bursts (sGRBs) are generally thought to result from themerger of two neutron stars or the merger of a neutron star with a black hole.It is becoming standard practise to model these mergers with hydrodynamicalsimulations that employ equations of state that are derived, for example, fordetermining the behaviour of matter in core-collapse supernovae (CCSNe), andwhich therefore make use of the assumption that the matter is hot and innuclear statistical equilibrium (NSE). In this Letter we draw attention to thefact that the hydrodynamical timescale (roughly the gravitational timescale ofthe neutron star) may be several orders of magnitude shorter than the timescaleon which such equilibrium can be re-established in the tidal debris ejectedduring a sGRB, and that on the initial decompression timescales the unshockedtidal ejecta may remain sufficiently cool that the employed equations of stateare not appropriate for modelling the dynamics of this part of the flow. Ontimescales short compared with the timescale on which NSE can be(re)established, the equation of state can remain relatively stiff and thus thestream of tidal debris can remain narrow and vulnerable to gravitationalinstability, as has recently been suggested. These findings suggest thatestimates of the type and abundances of heavy elements formed in shortgamma-ray bursts need to be revisited. We suggest that the most direct methodof testing the physical and dynamical properties of tidal ejecta in sGRBs willcome from modelling of their light curves, which provides the cleanest sourceof information on the system dynamics.

Short gamma-ray bursts and the decompression of neutron star matter in tidal streams