科研之友
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摘要
Most of supernova- originating presolar grains, such as silicon carbide type X (SiC X) and low-density graphite, show excesses of Si-28. Some of them also indicate evidence for the original presence of short-lived nuclei Ti-44. In order to reproduce isotopic and elemental signatures of these grains, large-scale heterogeneous mixing in supernova ejecta is required. I investigate supernova mixtures that reproduce as many isotopic ratios as possible of 18 individual SiC X and 26 individual low-density graphite grains. The supernova ejecta are divided into seven layers, i.e., the Ni, Si/S, O/Si, O/Ne, C/O or O/C, He/C, and He/N layers. Then, mixtures indicating isotopic ratios with small differences from those of individual single grains are sought. Mixtures reproducing five isotopic ratios in 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 are obtained for three SiC grains. For 10 SiC X grains for which five, four, or three isotopic ratios have been measured, the mixtures reproducing all measured isotopic ratios are also found. For 20 low-density graphite grains, the mixtures reproducing six isotopic ratios in C-12/C-13, N-14/N-15, O-16/O-17, O-16/O-18, Al-26/Al-27, Si-29/Si-28, Si-30/Si-28, and Ti-44/Ti-48 are obtained. The mixing ratios of each mixture strongly depend on reproduced isotopic ratios. The main component of most mixtures is one of the Ni, He/C, and He/N layers. The carbon-to-oxygen ratio of the mixtures is larger than unity for most cases. The ratios of Fe/C and Fe/Si in the mixtures have large varieties. Most of the mixtures for low-density graphite indicate Fe/Si larger than unity. The elemental signatures of the mixtures will constrain grain formation in supernovae.
- 出版日期2007-9-10
- 单位中国科学院国家天文台
