Magnetic Protoneutron Star Winds and r-Process Nucleosynthesis

Because of their neutron richness and association with supernovae, postexplosion 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_solar and coordinate radius of 10 km. These models fail variously; 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 >~6×10¹⁴ G increase the asymptotic entropy sufficiently for robust r-process nucleosynthesis.