CONVECTIVE-REACTIVE PROTON-¹²C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS

Abstract

Depending on mass and metallicity as well as evolutionary phase, stars\noccasionally experience convective-reactive nucleosynthesis episodes. We\nspecifically investigate the situation when nucleosynthetically unprocessed,\nH-rich material is convectively mixed with a He-burning zone, for example in\nconvectively unstable shell on top of electron-degenerate cores in AGB stars,\nyoung white dwarfs or X-ray bursting neutron stars. Such episodes are\nfrequently encountered in stellar evolution models of stars of extremely low or\nzero metal content [...] We focus on the convective-reactive episode in the\nvery-late thermal pulse star Sakurai's object (V4334 Sagittarii). Asplund etal.\n(1999) determined the abundances of 28 elements, many of which are highly\nnon-solar, ranging from H, He and Li all the way to Ba and La, plus the C\nisotopic ratio. Our simulations show that the mixing evolution according to\nstandard, one-dimensional stellar evolution models implies neutron densities in\nthe He that are too low to obtain a significant neutron capture nucleosynthesis\non the heavy elements. We have carried out 3D hydrodynamic He-shell flash\nconvection [...] we assume that the ingestion process of H into the He-shell\nconvection zone leads only after some delay time to a sufficient entropy\nbarrier that splits the convection zone [...] we obtain significantly higher\nneutron densities (~few 10^15 1/cm^3) and reproduce the key observed abundance\ntrends found in Sakurai's object. These include an overproduction of Rb, Sr and\nY by about 2 orders of magnitude higher than the overproduction of Ba and La.\nSuch a peculiar nucleosynthesis signature is impossible to obtain with the\nmixing predictions in our one-dimensional stellar evolution models. [...] We\ndetermine how our results depend on uncertainties of nuclear reaction rates,\nfor example for the C13(\\alpha, n)O16 reaction.\n