Tidal disruption events (TDEs) occur when stars are ripped apart 1,2 by massive black holes and result in highly luminous, multi-wavelength flares 3–5. Optical–ultraviolet observations 5–7 of TDEs contradict simple models of TDE emission 2,8, but the debate between alternative models (for example, shock power 9,10 or reprocessed accretion power 11–16) remains unsettled, as the dynamic range of the problem has so far prevented ab initio hydrodynamical simulations 17. Consequently, past simulations have resorted to unrealistic parameter choices 10,12,18–21, artificial mass injection schemes 22,23 or very short run-times 24. Here we present a three-dimensional radiation-hydrodynamic simulation of a TDE flare from disruption to peak emission, with typical astrophysical parameters. At early times, shocks near pericentre power the light curve and a previously unknown source of X-ray emission, but circularization and outflows are inefficient. Near peak light, stream–disk shocks efficiently circularize returning debris, power stronger outflows and reproduce observed peak optical–ultraviolet luminosities 25,26. Peak emission in this simulation is shock-powered, but upper limits on accretion power become competitive near peak light as circularization runs away. This simulation shows how deterministic predictions of TDE light curves and spectra can be calculated using moving-mesh hydrodynamics algorithms.
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