The physics of proto-neutron star winds: Implications for r-process nucleosynthesis

We solve the general-relativistic steady-state eigenvalue problem of neutri no-driven proto-neutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and compos ition profiles and explore the systematics and structures generic to such a wind for a variety of proto-neutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and neutron-to-seed ratio in the g eneral relativistic framework essential in assessing this site as a candida te for r-process nucleosynthesis. Generally, we find that for a given mass outflow rate ((M) over dot), the dynamical cal timescale of the wind is sig nificantly shorter than previously thought. We argue against the existence or viability of a high entropy (greater than or similar to 300 per k(B) per baryon), long dynamical timescale r-process epoch. In support of this conc lusion, we model the proto-neutron star cooling phase, calculate nucleosynt hetic yields in our steady-state profiles, and estimate the integrated mass loss. We find that transonic winds enter a high-entropy phase only with ve ry low (M) over dot (less than or similar to 1 x 10(-9) M. s(-1)) and extre mely long dynamical timescale (tau (rho) greater than or similar to 0.5 s). Our results support the possible existence of an early r-process epoch at modest entropy (similar to 150) and very short dynamical timescale, consist ent in our calculations with a very massive or very compact proto-neutron s tar that contracts rapidly after the preceding supernova. We explore possib le modifications to our models, which might yield significant r-process nuc leosynthesis generically. Finally, we speculate on the effect of fallback a nd shocks on both the wind physics and nucleosynthesis. We find that a term ination or reverse shock in the wind, but exterior to the wind sonic point, may have important nucleosynthetic consequences. The potential for the r-p rocess in proto-neutron star winds remains an open question.