The Physics of Protoneutron Star Winds: Implications for R-process Nucleosynthesis

We solve the general-relativistic steady-state eigenvalue problem of neutrino-driven protoneutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and composition profiles and explore the systematics and structures generic to such a wind for a variety of protoneutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and neutron-to-seed ratio in the general relativistic framework essential in assessing this site as a candidate for r-process nucleosynthesis. Generally, we find that for a given mass outflow rate (Ṁ), the dynamical timescale of the wind is significantly shorter than previously thought. We argue against the existence or viability of a high entropy (& 300 per kB per baryon), long dynamical timescale r-process epoch. In support of this conclusion, we model the protoneutron star cooling phase, calculate nucleosynthetic yields in our steady-state profiles, and estimate the integrated mass loss. We find that transonic winds enter a high entropy phase only with very low Ṁ (. 1×10−9 M⊙ s−1) and extremely long dynamical timescale (τρ & 0.5 seconds). Our results support the possible existence of an early r-process epoch at modest entropy (∼ 150) and very short dynamical timescale, consistent in our calculations with a very massive or very compact protoneutron star that contracts rapidly after the preceding supernova. We explore possible modifications to our models, which might yield significant r-process nucleosynthesis generically. Finally, we speculate on the effect of fallback and shocks on both the wind physics and nucleosynthesis. We find that a termination or reverse shock in the wind, but exterior to the wind sonic point, may have important nucleosynthetic consequences. The potential for the r-process in protoneutron star winds remains an open question. Subject headings: Winds, neutrinos, supernovae, neutron stars, nucleosynthesis