MIXING IN ZERO- AND SOLAR-METALLICITY SUPERNOVAE

Two-dimensional simulations of mixing and fallback in nonrotating massive stars have been carried out using realistic initial models for the presupernova star and assuming standard spherically symmetric explosions of 1.2 x 10(51) erg. Stars of 15 and 25 M-circle dot with both primordial and solar composition were modeled. The zero-metallicity supernova progenitors were compact blue stars, and the amount of Rayleigh-Taylor induced mixing in them was greatly reduced compared with what was seen in the red supergiants with solar metallicity. The compact zero-metal stars also experienced more fallback than their solar-metallicity counterparts. As a result, the ejected nucleosynthesis from the two populations was very different. For the simple explosion model assumed, low-metallicity stars ejected too little iron and intermediate-mass elements even to explain the abundance patterns in the most iron-poor stars found to date, suggesting that some important ingredient is missing. Rotation is likely to alter these conclusions by producing a greater fraction of red supergiants among Population III stars. The velocities of the heavy elements in all models considered-both red and blue supergiants-were less than observed in SN 1987A, suggesting that, at least occasionally, asymmetric aspects of the explosion mechanism and fallback play a major role in mixing.