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We study mass ejection from accretion disks around newly-formed hypermassiveneutron stars (HMNS). Standard kilonova model fits to GW170817 require at leasta lanthanide-poor ('blue') and lanthanide-rich ('red') component. The existenceof a blue component has been used as evidence for a HMNS remnant of finitelifetime, but average disk outflow velocities from existing long-term HMNSsimulations fall short of the inferred value ($\sim 0.25c$) by a factor of$\sim 2$. Here we use time-dependent, axisymmetric hydrodynamic simulations ofHMNS disks to explore the limits of the model and its ability to account forobservations. For physically plausible parameter choices compatible withGW170817, we find that hydrodynamic models that use shear viscosity totransport angular momentum cannot eject matter with mass-averaged velocitieslarger than $\sim 0.15c$. While outflow velocities in our simulations canexceed the asymptotic value for a steady-state neutrino-driven wind, theincrease in the average velocity due to viscosity is not sufficient. Therefore,viscous HMNS disk winds cannot reproduce by themselves the ejecta propertiesinferred from multi-component fits to kilonova light curves from GW170817.Three possible resolutions remain feasible within standard merger ejectachannels: more sophisticated radiative transfer models that allow for photonreprocessing between ejecta components, inclusion of magnetic stresses, orenhancement of the dynamical ejecta. We provide fits to our disk outflow modelsonce they reach homologous expansion.

Hypermassive Neutron Star Disk Outflows and Blue Kilonovae