Numerical Analyses of Isotopic Ratios of Presolar Grains from Supernovae

Abstract

Presolar grains have large isotopic anomalies revealing the traces of nucleosynthesis at their birth, and thus they are considered to originate from stellar outflows and supernova ejecta. These grains have been extracted from primitive meteorites, and the isotopic ratios of each individual grain have been analyzed in the laboratory. Several isotopic signatures of SiC X and low-density graphite grains indicate an origin in supernovae. We investigate the isotopic ratios of Type II supernova ejecta with large-scale mixing so as to reproduce the ratios of as many ratios from supernovae as possible. We pursued the evolution and the supernova explosion of a massive star using a 4 M. He star model and calculated detailed nucleosynthesis by postprocessing. We then show how many isotopic ratios of each individual grain agree with those of the mixtures of the supernova ejecta under appropriate mixing ratios. We assume two cases of mixtures with artificial mixing ratios: one consists of Si/S, O/Ne, He/C, and He/N layers, and the other consists of Ni, Si/S, He/C, and He/N layers. We select the isotopic ratios of C-12/C-13, N-14/N-15, Al-26/Al-27, Si-29/Si-28, Si-30/Si-28, and Ti-44/Ti-48 from eleven SiC X grains and four low-density graphite grains that contain evidence for the original presence of the short-lived isotope Ti-44. For one SiC X grain and one low-density graphite grain, four isotopic ratios are reproduced by the corresponding mixtures. For seven SiC X grains and three graphite grains, three isotopic ratios agree with those of the mixtures. Most mixtures that reproduce the isotopic ratios of the grains have a large contribution from the He/N layer. The characteristics of the mixtures are found to be 0.01 < 10 and C/O > 0.1. For C/O < 0.1, the isotopic ratios of most grains are not reproduced.