We demonstrate a qualitatively new aspect of the dynamics of gamma-ray burst (GRB) fireballs: the development of a substantial dispersion in the proton component in fireballs in which neutron decoupling occurs and is sufficiently pronounced. This effect depends sensitively on the neutron to proton ratio in the fireball, becoming more dramatic with increasing neutron excess. Simple physical arguments and transport calculations indicate that the dispersion in the Lorentz factor of the protons can be of the order of the final mean Lorentz factor of the fireball. We show how plasma instabilities could play an important role in the evolution of the fireball and how they might ultimately govern the development of such a velocity dispersion in the proton component. The role of these instabilities in setting or diminishing a proton Lorentz factor dispersion represents a new and potentially important venue for the study of plasma instabilities. Significant dispersion in the proton velocities translates into fewer protons attaining the highest Lorentz factors. This is tantamount to a reduction in the total energy required to attain a given Lorentz factor for the highest energy protons. As well, a velocity dispersion in the proton component can have consequences for the electromagnetic and neutrino signature of GRB's.

We calculate and discuss the light-element freezeout and nonthermal reaction nucleosynthesis in high-entropy winds and fireballs for broad ranges of entropy per baryon, dynamic timescales characterizing relativistic expansion, and neutron-to-proton ratios. With conditions characteristic of gamma-ray bursts (GRBs), we find that deuterium production can be prodigious, with final abundance values 2H/H≳2%, depending on the fireball isospin, late-time dynamics, and the effects of neutron-decoupling-induced high-energy nonthermal nuclear reactions. This implies that there could potentially be detectable local enhancements in the deuterium abundance associated with GRB events.

We point out that the baryon loading problem in gamma-ray burst (GRB) models can be ameliorated if a significant fraction of the baryons which inertially confine the fireball is converted to neutrons. A high neutron fraction can result in a reduced transfer of energy from relativistic light particles in the fireball to baryons. The energy needed to produce the required relativistic flow in the GRB is consequently reduced, in some cases by orders of magnitude. A high neutron-to-proton ratio has been calculated in neutron star-merger fireball environments. Significant neutron excess also could occur near compact objects with high neutrino fluxes.

We study steady state winds from compact objects in the regime where the wind velocity at infinity is ultrarelativistic. This may have relevance to some models of gamma-ray bursts (GRBs). Particular attention is paid to the case in which neutrinos provide the heating. Unless the neutrino luminosity is very large, L > 1054 ergs s-1, the only allowed steady state solutions are those where energy deposition is dominated by neutrino-antineutrino annihilation at the sonic point. In this case, the matter temperature near the neutron star surface is low, less than 1 MeV for typical neutrino luminosities. This is in contrast to the case for subrelativistic winds discussed in the context of supernovae, where the matter temperature near the neutron star approximates the temperature characterizing the neutrinos. We also investigate the setting of the neutron-to-proton ratio (N/P) in these winds and find that only for large (> 10 MeV) electron-neutrino or electron-antineutrino temperatures is N/P entirely determined by neutrino capture. Otherwise, N/P retains an imprint of conditions in the neutron star.