Nucleosynthesis and Short Gamma-Ray Burst Central Engines

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Massive accretion disks may form from the merger of neutron star (NS)-NS or black hole-NS binaries, or following the accretion-induced collapse (AIC) of a white dwarf. These disks, termed `hyper-accreting' due to their accretion rates up to several solar masses per second, may power the relativistic jets responsible for short duration gamma-ray bursts. Using 1D time-dependent calculations of hyper-accreting disks, I show that a generic consequence of the disk's late-time evolution is the development of a powerful outflow, powered by viscous heating and the recombination of free nuclei into Helium. These outflows - in additional to any material dynamically-ejected during the merger - synthesize heavy radioactive elements as they expand into space. Nuclear heating from the r-process is not yet incorporated in merger simulations, yet has important consequences both for the dynamics of late `fall-back' accretion and in powering a supernova-like transient (`kilonova') 1 day following the merger or AIC.