Because of their neutron-richness and association with supernovae , post-explosion protoneutron star winds are thought to be a likely astrophysical site for rapid neutron capture nucleosynthesis ( the r -process ) .
However , the most recent models of spherical neutrino-driven protoneutron star winds do not produce robust r -process nucleosynthesis for ‘ canonical ’ neutron stars with a gravitational mass of 1.4 M _ { \odot } and coordinate radius of 10 km .
These models fail variously ; either the flow entropy is too low , the electron fraction is too high , or the dynamical expansion timescale is too long .
To date , no models have included the effects of an ordered dipole magnetic field .
We show that a strong magnetic field can trap the outflow in the neutrino heating region , thus leading to much higher matter entropy .
We estimate both the trapping timescale and the resulting entropy amplification .
For sufficiently large energy deposition rates , the trapped matter emerges dynamically from the region of closed magnetic field lines and escapes to infinity .
We find that ordered dipoles with surface fields of \gtrsim 6 \times 10 ^ { 14 } G increase the asymptotic entropy sufficiently for robust r -process nucleosynthesis .