科研之友
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摘要
We have conducted a NanoSIMS ion imaging survey of about 1800 presolar silicon carbide (SiC) grains from the Murchison meteorite. A total of 21 supernova (SN) X grains, two SN C grains, and two putative nova grains were identified. Six particularly interesting grains, two X and C grains each and the two putative nova grains were subsequently studied in greater detail, namely, for C-, N-, Mg-Al-, Si-, S-, and Ca-Ti-isotopic compositions and for the initial presence of radioactive Al-26 (half life 716,000 yr), Si-32 (half life 153 yr), and Ti-44 (half life 60 yr). Their isotope data along with those of three X grains from the literature were compared with model predictions for 15 M-circle dot and 25M(circle dot) Type II supernovae (SNe). The best fits were found for 25M(circle dot) SN models that consider for the He shell the temperature and density of a 15 M-circle dot SN and ingestion of H into the He shell before the explosion. In these models a C-and Si-rich zone forms at the bottom of the He burning zone (C/Si zone). The region above the C/Si zone is termed the O/nova zone and exhibits the isotopic fingerprints of explosive H burning. Satisfactory fits of measured C-, N-, and Si-isotopic compositions and of Al-26/Al-27 ratios require small-scale mixing of matter originating from a region extending over 0.2 M-circle dot for X and C grains and over 0.4 M-circle dot for one of the putative nova grains, involving matter from a thin Si-rich layer slightly below the C/Si zone, the C/Si zone, and the O/nova zone. Simultaneous fitting of N-14/N-15 and Al-26/Al-27 requires a C-N fractionation of a factor of 50 during SiC condensation. This leads to preferential incorporation of radioactive C-14 (half life 5700 yr) over directly produced N-14 and can account for the N-14/N-15 along with Al-26/Al-27 ratios as observed in the SiC grains. The good fit for one of the putative nova grains along with its high Al-26/Al-27 points towards a SN origin and supports previous suggestions that some grains classified as nova grains might be from SNe. Apparent problems with the small-scale mixing scheme considered here are C/O ratios that are mostly < 1 if C-, N-, and Si-isotopic compositions and Al-26/Al-27 ratios are simultaneously matched, underproduction of Si-32, and overproduction of Ti-44. This confirms the limitations of one-dimensional hydrodynamical models for H ingestion and stresses the need to better study the convective-boundary mixing mechanisms at the bottom of the convective He shell in massive star progenitors. This is crucial to define the effective size of the C/Si zone formed by the SN shock. The comparison between the Si isotope data of the SN grains and the models gives a hint that the predicted Si-30 is too high at the bottom of the He burning shell.
- 出版日期2018-1-15
