On the Conditions for Neutron-rich Gamma-ray Burst Outflows

We calculate the structure and neutron content of neutrino-heated magnetohydrodynamic winds driven from the surface of newly-formed magnetars (“proto-magnetars”) and from the midplane of hyper-accreting disks, two of the possible central engines for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the surface of proto-magnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p≫ 1). If this substantial free neutron excess is preserved to large radii in ultra-relativistic outflows, several important observational consequences may result. Weak interaction processes, however, can drive n/p to ∼ 1 in the nondegenerate regions that obtain just above the surfaces of NDAFs and proto-magnetars. Our calculations show that mildly relativistic (Lorentz factor Γ . 10) neutron-rich outflows from NDAFs are possible in the presence of a strong poloidal magnetic field. However, neutron-rich winds possess a minimum mass-loss rate that likely precludes simultaneously neutron-rich and ultra-relativistic (Γ & 100) NDAF winds accompanying a substantial accretion power. In contrast, protomagnetars are capable of producing neutron-rich long-duration GRB outflows ∼ 10 − 30 seconds following core bounce for sub-millisecond rotation periods; such outflows would, however, accompany only extremely energetic events, in which the GRB + SN energy budget exceeds ∼ 4×1052 ergs. Neutron-rich highly relativistic outflows may also be produced during some short-duration GRBs by geometrically thick accretion disks formed from compact object mergers. The implications for r-process nucleosynthesis, optical transients due to non-relativistic neutron-rich winds, and Nickel production in proto-magnetar and NDAF winds are also briefly discussed. Subject headings: stars: neutron — stars: winds, outflows — supernovae: general — gamma rays: bursts — stars: magnetic fields — accretion, accretion disks — nuclear reactions, nucleosynthesis, abundances