Observational evidence for mixing and dust condensation in core-collapse supernovae

Abstract

Recent findings of isotopic anomalies of 44Ca (the decay product of 44Ti) and the enhanced ratio of 28Si/30Si in SiC grains X, TiC subgrains, and graphite dust grains within primitive meteorites provides strong evidence that these presolar grains came from core-collapse supernovae. The chemical composition of the presolar grains requires macroscopic mixing of newly nucleosynthesized elements from explosive silicon burning at the innermost zone of the ejecta to higher velocities where C exists and where C/O>1 in either the outer edge of the oxygen zone or in the He-C zone. To date, the only core-collapse supernova observed to form dust is the brightest supernova of the past four centuries, SN1987A in the Large Magellanic Cloud. Observations of SN1987A confirm large scale macroscopic mixing occurs in the explosions of massive stars. Rayleigh-Taylor instabilities macroscopically mix most of the ejecta into regions which are still chemically homogeneous and which cool with different time scales. Only small clumps in the ejecta are microscopically mixed. Observations show that dust condensed in the ejecta of SN1987A after ∼500 days in the Fe-rich gas. Neither silicates nor SiC grains were seen in the dust emission spectrum of SN1987A. SN1987A, the Rosetta Stone of core-collapse supernovae, shows that while the mixing required to explain presolar grains occurs, the rapid cooling of the Fe zone and the sustained high temperatures of the O-Si, O-C, and He-C zones favor the formation of iron-rich rather than oxygen-or carbon-rich grains.