# Spiral-wave wind for the blue kilonova

## TL;DR

This paper proposes a new mechanism involving spiral density waves in neutron star merger remnants that can produce the blue kilonova observed in GW170817, explaining its early brightness and nucleosynthesis.

## Contribution

The study introduces a novel spiral-wave wind mechanism from neutron star merger remnants, supported by detailed numerical simulations, to explain the blue kilonova and r-process element production.

## Key findings

- Spiral density waves generate a wind of about 0.01 solar masses at 0.2c.
- Ejected material has electron fraction mostly above 0.25.
- The combined ejecta can reproduce observed light curves and r-process abundances.

## Abstract

The AT2017gfo kilonova counterpart of the binary neutron star merger event GW170817 was characterized by an early-time bright peak in optical and UV bands. Such blue kilonova is commonly interpreted as a signature of weak $r$-process nucleosynthesis in a fast expanding wind whose origin is currently debated. Numerical-relativity simulations with microphysical equations of state, approximate neutrino transport, and turbulent viscosity reveal a new mechanism that can power the blue kilonova. Spiral density waves in the remnant generate a characteristic wind of mass ${\sim}10^{-2}~M_{\odot}$ and velocity ${\sim}0.2$c. The ejected material has electron fraction mostly distributed above $0.25$ being partially reprocessed by hydrodynamic shocks in the expanding arms. The combination of dynamical ejecta and spiral-wave wind can account for solar system abundances of $r$-process elements and early-time observed light curves.