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 ("protomagnetars") and from the midplane of hyperaccreting disks, two of the possible central engines for GRBs and hyperenergetic SNe. Both the surface of protomagnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p gg 1). If this substantial free neutron excess is preserved to large radii in ultrarelativistic 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 protomagnetars. Our calculations show that mildly relativistic (Lorentz factor Γ lesssim 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 ultrarelativistic (Γ gtrsim 100) NDAF winds accompanying a substantial accretion power. In contrast, protomagnetars are capable of producing neutron-rich long-duration GRB outflows ~10-30 s following core bounce for submillisecond 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 nonrelativistic neutron-rich winds, and nickel production in protomagnetar and NDAF winds are also briefly discussed.