一颗原始陨石中古老的后AGB星遗迹
Relics of ancient post-AGB stars in a primitive meteorite
摘要：
石墨是在原始陨石中发现的许多太阳系前的星周冷凝物之一。虽然低密度石墨颗粒的同位素组成表明起源于核心坍缩超新星，但一些高密度颗粒在C，Ca和Ti中具有极端的同位素异常，这不能用渐近巨星分支（AGB）恒星或理论超新星模型的包络预测来解释。Ca和Ti同位素异常，然而，匹配的AGB星的He壳丰度的预测。在这项研究中，我们表明，C，Ca和Ti同位素异常是一致的核合成预测的H-摄入阶段在一个非常晚的热脉冲（VLTP）事件后AGB星。这些颗粒中的低$^{12}$ C/ $^{13}$ C同位素比是大量的$^{12}$ C有效捕获VLTP期间摄入的质子的结果。非常高的中子密度$\sim 10^{15}$ cm $^{-3}$，典型的$i$ -过程中，实现在此阶段的后AGB星。在某些石墨颗粒中大量的$^{42,43,44}$ Ca过量是VLTP期间中子俘获核合成的指示。VLTP核合成计算与石墨数据的比较还表明，Ti同位素比的明显异常是由于$^{46,48}$ Ca的巨大贡献造成的，而在测量过程中无法从同量异位素$^{46,48}$ Ti中分辨出来。我们的结论是，前太阳石墨颗粒与中度极端钙和钛同位素异常起源于后AGB恒星，遭受一个非常晚的热脉冲。
英文摘要：
Graphite is one of the many presolar circumstellar condensate species found in primitive meteorites. While the isotopic compositions of low-density graphite grains indicate an origin in core-collapse supernovae, some high-density grains have extreme isotopic anomalies in C, Ca and Ti, which cannot be explained by envelope predictions of asymptotic giant branch (AGB) stars or theoretical supernova models. The Ca and Ti isotopic anomalies, however, match the predictions of He-shell abundances in AGB stars. In this study, we show that the C, Ca, and Ti isotopic anomalies are consistent with nucleosynthesis predictions of the H-ingestion phase during a very late thermal pulse (VLTP) event in post-AGB stars. The low $^{12}$C/$^{13}$C isotopic ratios in these grains are a result of abundant $^{12}$C efficiently capturing the protons that are being ingested during the VLTP. Very high neutron densities of $\sim 10^{15}$ cm$^{-3}$, typical of the $i$-process, are achieved during this phase in post-AGB stars. The large $^{42,43,44}$Ca excesses in some graphite grains are indicative of neutron capture nucleosynthesis during VLTP. The comparison of VLTP nucleosynthesis calculations to the graphite data also indicate that apparent anomalies in the Ti isotopic ratios are due to large contributions from $^{46,48}$Ca, which cannot be resolved from the isobars $^{46,48}$Ti during the measurements. We conclude that presolar graphite grains with moderate to extreme Ca and Ti isotopic anomalies originate in post-AGB stars that suffer a very late thermal pulse.
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