  Tidal disruption events occur when stars are ripped apart by massive black
holes, and result in highly luminous, multi-wavelength flares. Optical/UV
observations of tidal disruption events (TDEs) contradict simple models of TDE
emission, but the debate between alternative models (e.g. shock power or
reprocessed accretion power remains unsettled, as the dynamic range of the
problem has so far prevented ab initio hydrodynamical simulations.
Consequently, past simulations have resorted to unrealistic parameter choices,
artificial mass injection schemes or very short run-times. Here we present a 3D
radiation-hydrodynamic simulation of a TDE flare from disruption to peak
emission, with typical astrophysical parameters. At early times, shocks near
pericenter power the light curve and a novel 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/UV luminosities. 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.
