Origin of Anomalous Xe-H in Nanodiamond Stardust

Still today, the nucleosynthesis origin of Xe-H in presolar nanodiamonds is far from understood. Historically, possible explanations were proposed by a secondary "neutron-burst" process occurring in the He- or C/O-shells of a type-II supernova (SN-II), which are, however, not fully convincing in terms of modern nucleosynthesis conditions. Therefore, we have investigated Xe isotopic abundance features that may be diagnostic for different versions of a classical, primary r-process in high-entropy-wind (HEW) ejecta of core-collapse SN-II. We report here on parameter tests for non-standard r-process variants, by varying electron abundances (Y$_e$), ranges of entropies (S) and expansion velocities (V$_{exp}$) with their correlated neutron-freezeout times ($τ$(freeze)) and temperatures (T$_9$(freeze)). From this study, we conclude that a best fit to the measured Xe-H abundance ratios $^i$Xe/$^{136}$Xe can be obtained with the high-S "main" component of a "cold" r-process variant.

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