Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows - I. Spherically symmetric hydrodynamic simulations

We investigate the behavior and consequences of the reverse shock that terminates the supersonic expansion of the baryonic wind which is driven by neutrino heating of the surface of ( non-magnetized) new-born neutron stars in supernova cores. To this end we perform long-time hydrodynamic simulations in spherical symmetry. In agreement with previous relativistic wind studies, we find that the neutrino-driven outflow accelerates to supersonic velocities and in case of a compact, similar to 1.4 M-circle dot ( gravitational mass) neutron star with a radius of about 10 km, the wind reaches entropies of about 100 k(B) per nucleon. The wind, however, is strongly influenced by the environment of the supernova core. It is decelerated and shock-heated abruptly by a termination shock that forms when the supersonic outflow collides with the slower preceding supernova ejecta. The radial position of this reverse shock varies with time and depends on the strength of the neutrino wind and the explosion conditions in progenitor stars with different masses and structure. Its basic properties and behavior can be understood by simple analytic considerations. We demonstrate that the entropy of the matter going through the reverse shock can increase to a multiple of the asymptotic wind value. Seconds after the onset of the explosion it therefore can exceed 400 kB per nucleon in low-mass progenitors around 10 M-circle dot, where the supernova shock and the reverse shock propagate outward quickly. The temperature of the shocked wind has typically dropped to about or less than 109 K, and density and temperature in the shock-decelerated matter continue to decrease only very slowly. For more massive progenitors with bigger and denser metal cores, the explosion expands more slowly so that the termination shock stays at smaller radii and affects the wind at higher temperatures and densities. In this case the termination shock might play a non- negligible, strongly time- and progenitor-dependent role in discussing supernova nucleosynthesis.