Classically, the explosive α-capture at the bottom of the former convective He-shell activates a neutron burst peaking up to 1018–20 neutrons cm−3, produced by the 22Ne(α, n)25Mg reaction. This nucleosynthesis process was called the n-process (Blake & Schramm 1976; Thielemann, Arnould & Hillebrandt 1979). Much later, its signature was identified in heavy elemental abundances measured in presolar SiC-X grains such as large excesses of 58Fe, 88Sr, 96Zr, 95, 97Mo, and 138Ba (e.g. Meyer, Clayton & The 2000; Pellin et al. 2006; Pignatari et al. 2018) and in the production of the radioactive isotope 32Si (Pignatari et al. 2015). However, this scenario has been discounted as a way to explain the r-process because it could not reproduce the r-process abundance pattern in the Solar system (Blake et al. 1981). The presence of H in the He-shell during the SN explosion may lead to a suppression of the n process, where it becomes more probable to destroy 22Ne by proton capture than by the 22Ne(α, n)25Mg reaction (Pignatari et al. 2015). The production of 44Ti and 40Ca in the C/Si zone along with the possible strong depletion of 40Ca by neutron capture predicts abundances consistent with the observation of large 44Ca/40Ca in the C-rich grains discussed (Pignatari et al. 2013a). It is still matter of debate if in presolar grains the abundance signatures of H-ingestion coexist with the neutron-capture signature of the n-process. Indeed, the n-process activation would be compatible with the 30Si- and 32S-enhancements measured in in some putative nova grains (Liu et al. 2016; Hoppe et al. 2018), and with the 32Si- and 50Ti-enhancements found in a sample of SiC–AB grains (Liu et al. 2017a). On the other hand, measurements of the Sr, Mo, and Ba isotopic abundances in SiC–AB grains seem to be compatible with the pre-explosive s-process abundances in the He shell, and not with the n-process (Liu et al. 2018b). In the first case, major asymmetries triggered by the H ingestion in the pre-explosive He shell structure would be required in order to explain for the same single grain the signatures of both the n-process and of a late H-ingestion event leaving H behind in the former He shell material. While this may be a natural expectation from multidimensional simulations, it cannot be captured by 1D stellar simulations. Additionally, those abundance signatures of high neutron-density exposures could also be the effect of a local activation of the intermediate neutron-capture process (i-process; Cowan & Rose 1977), following the H ingestion event. The i-process has been proposed as a nucleosynthesis source active in massive stars at low metallicities (e.g. Roederer et al. 2016; Banerjee, Qian & Heger 2018; Clarkson, Herwig & Pignatari 2018), but it also has been proposed as a source of anomalous signatures in presolar grains for Ti isotopes (HD graphites; Jadhav et al. 2013), Ba isotopes (in some mainstream SiC grains; Liu et al. 2014) and 32Si-enrichments (in SiC AB grains; Fujiya et al. 2013). In order to verify all of these different scenarios, new abundance observations are required for more presolar grains of different types.
Citation: Pignatari et al. 2013a
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