Si Isotopic Ratios in Supernovae for Presolar Grains

Abstract

Primitive meteorites contain presolar grains that are considered to have traces of the nucleosynthesis at their birth as large isotopic anomalies. The isotopic signatures are important to identify the origin of the grains. Supernova-originating presolar grains, such as SiC type X and low-density graphite, have also been identified by the excesses of 28Si with respect to the solar Si isotopic ratios. At the same time, most of them are depleted in 30Si rather than in 29Si compared to the solar Si ratios. A small number of them show 29Si deficits. We investigate the Si isotopic ratios of supernova ejecta with different-mass progenitors considering inhomogeneous mixing. The mixture components consist of four layers of the supernova ejecta: the Ni, Si/S, He/C, and He/N layers. Detailed nucleosynthesis during stellar evolution and supernova explosion is calculated using 3.3, 4, 6, and 8 M☉ He star models. Supernova explosions for all of the stellar models and a hypernova explosion for the 8 M☉ He star model are considered. The 30Si deficits with respect to solar ratios found in most grains are explained by the mixtures of the 3.3 and 4 M☉ supernova models and the 8 M☉ hypernova model. The 29Si deficits are explained by the mixtures of the 6 and 8 M☉ supernova models. Although the Si abundance in the Ni layer is small, the Ni layer is important to reproduce the Si isotopic ratios of the grains. The Si isotopic ratios in the Ni layer largely depend on the stellar mass, because a shorter explosion timescale is more favorable to the production of 29Si than 30Si. Most presolar grains from supernovae with deficits of 30Si rather than of 29Si compared to the solar Si isotopic ratios would be from less massive supernovae whose progenitor masses are smaller than ~15 M☉ and hypernovae whose progenitor masses are heavier than ~20 M☉.