  We study mass ejection from accretion disks around newly-formed hypermassive
neutron stars (HMNS). Standard kilonova model fits to GW170817 require at least
a lanthanide-poor ('blue') and lanthanide-rich ('red') component. The existence
of a blue component has been used as evidence for a HMNS remnant of finite
lifetime, but average disk outflow velocities from existing long-term HMNS
simulations fall short of the inferred value ($\sim 0.25c$) by a factor of
$\sim 2$. Here we use time-dependent, axisymmetric hydrodynamic simulations of
HMNS disks to explore the limits of the model and its ability to account for
observations. For physically plausible parameter choices compatible with
GW170817, we find that hydrodynamic models that use shear viscosity to
transport angular momentum cannot eject matter with mass-averaged velocities
larger than $\sim 0.15c$. While outflow velocities in our simulations can
exceed the asymptotic value for a steady-state neutrino-driven wind, the
increase in the average velocity due to viscosity is not sufficient. Therefore,
viscous HMNS disk winds cannot reproduce by themselves the ejecta properties
inferred from multi-component fits to kilonova light curves from GW170817.
Three possible resolutions remain feasible within standard merger ejecta
channels: more sophisticated radiative transfer models that allow for photon
reprocessing between ejecta components, inclusion of magnetic stresses, or
enhancement of the dynamical ejecta. We provide fits to our disk outflow models
once they reach homologous expansion.
