Nucleosynthesis in neutrino-driven winds. I. The physical conditions

During the first 20 s of its life, the enormous neutrino luminosity of a neutron star drives appreciable mass loss from its surface. Previous investigations have shown that this neutrino-driven wind could be the site where the r-process occurs. The nucleosynthesis is sensitive to four physical parameters that characterize the wind: its mass-loss rate, the entropy per baryon, the electron fraction, and the dynamic timescale. Different authors, using numerical models for supernovae, have arrived at qualitatively different values for these key parameters. Here we derive their values analytically and test our analytic results by numerical calculations using an implicit hydrodynamic code. Employing our analytic and numerical methods, we also investigate how various factors can affect our results. The derived entropy typically falls short, by a factor of 2-3, of the value required to produce a strong r-process. Various factors that might give a higher entropy or a more rapid expansion in the wind are discussed.