  The neutron star (NS) merger GW170817 was followed over several days by
optical-wavelength ("blue") kilonova (KN) emission likely powered by the
radioactive decay of light r-process nuclei synthesized by ejecta with a low
neutron abundance (electron fraction Ye ~ 0.25-0.35). While the composition and
high velocities of the blue KN ejecta are consistent with shock-heated
dynamical material, the large quantity is in tension with the results of
numerical simulations. We propose an alternative ejecta source: the
neutrino-heated, magnetically-accelerated wind from the strongly-magnetized
hypermassive NS (HMNS) remnant. A rapidly-spinning HMNS with an ordered surface
magnetic field of strength B ~ 1-3e14 G and lifetime t_rem ~ 0.1-1 s can
simultaneously explain the velocity, total mass, and electron fraction of the
blue KN ejecta. The inferred HMNS lifetime is close to its Alfven crossing
time, suggesting global magnetic torques could be responsible for bringing the
HMNS into solid body rotation and instigating its gravitational collapse.
Different origins for the KN ejecta may be distinguished by their predictions
for the emission in the first hours after the merger, when the luminosity is
enhanced by heating from internal shocks; the latter are likely generic to any
temporally-extended ejecta source (e.g. magnetar or accretion disk wind) and
are not unique to the emergence of a relativistic jet. The same shocks could
mix and homogenizes the composition to a low but non-zero lanthanide mass
fraction, X_La ~ 1e-3, as advocated by some authors, but only if the mixing
occurs after neutrons are consumed in the r-process on a timescale >~ 1 s.
