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Abstract Presolar graphite grains carry the isotopic signatures of their parent stars. A significant fraction of presolar graphites show isotopic abundance anomalies relative to solar for elements such as O, Si, Mg, and Ca, which are compatible with nucleosynthesis in core-collapse supernovae (CCSNe). Therefore, they must have condensed from CCSN ejecta before the formation of the Sun. Their most puzzling abundance signature is the²²Ne-enriched component Ne-E(L), interpreted as the effect of the radioactive decay of²²Na (T_1/2= 2.6 yr). Previous works have shown that if H is ingested into the He shell and not fully destroyed before the explosion, the CCSN shock in the He-shell material produces large amounts of²²Na. Here we focus on such CCSN models, showing a radioactive²⁶Al production compatible with grain measurements, and analyze the conditions of²²Na nucleosynthesis. In these models,²²Na is mostly made in the He shell, with a total ejected mass varying between 2.6 × 10⁻³M_⊙and 1.9 × 10⁻⁶M_⊙. We show that such²²Na may already impact the CCSN light curve 500 days after the explosion, and at later stages it can be the main source powering the CCSN light curve for up to a few years before⁴⁴Ti decay becomes dominant. Based on the CCSN yields above, the 1274.53 keVγ-ray flux due to²²Na decay could be observable for years after the first CCSN light is detected, depending on the distance. This makes CCSNe possible sites to detect a²²Naγ-ray signature consistently with the Ne-E(L) component found in presolar graphites. Finally, we discuss the potential contribution from²²Na decay to the Galactic positron annihilation rate. 

Context.In their final stages before undergoing a core-collapse supernova, massive stars may experience mergers between internal shells where carbon (C) and oxygen (O) are consumed as fuels for nuclear burning. This interaction, known as a C-O shell merger, can dramatically alter the internal structure of the star, leading to peculiar nucleosynthesis and potentially influencing the supernova explosion and the propagation of the subsequent supernova shock. Aims.Our understanding of the frequency and consequences of C-O shell mergers remains limited. This study aims to identify, for the first time, early diagnostics in the stellar structure that lead to C-O shell mergers in more advanced stages. We also assess their role in shaping the chemical abundances in the most metal poor stars of the Galaxy. Methods.We analyzed a set of 209 stellar evolution models available in the literature, with different initial progenitor masses and metallicities. We then compared the nucleosynthetic yields from a subset of these models with the abundances of odd-Zelements in metal-poor stars. Results.We find that the occurrence of C-O shell mergers in stellar models can be predicted with a good approximation based on the outcomes of the central He burning phase, specifically, from the CO core mass (M_CO) and the¹²C central mass fraction (X_C12): 90% of models with a C-O merger have X_C12<0.277 and M_CO<4.90 M_⊙, with average values of M_CO= 4.02 M_⊙and X_C12= 0.176. The quantities X_C12and M_COare indirectly affected from several stellar properties, including the initial stellar mass and metallicity. Additionally, we confirm that the Sc-rich and K-rich yields from models with C-O mergers would solve the long-standing underproduction of these elements in massive stars. Conclusions.Our results emphasize the crucial role of C-O shell mergers in enriching the interstellar medium, particularly in the production of odd-Zelements. This highlights the necessity of further investigations to refine their influence on presupernova stellar properties and their broader impact on Galactic chemical evolution. 

Context.Theγprocess in core-collapse supernovae (CCSNe) can produce a number of neutron-deficient stable isotopes heavier than iron (pnuclei). However, current model predictions do not fully reproduce solar abundances, especially for^92, 94Mo and^96, 98Ru. Aims.We investigate the impact of different explosion energies and parametrizations on the nucleosynthesis ofpnuclei, by studying stellar models with different initial masses and different CCSN explosions. Methods.We compared thep-nucleus yields obtained using a semi-analytical method to simulate the supernova to those obtained using hydrodynamic models. We explored the effect of varying the explosion parameters on thep-nucleus production in two sets of CCSN models with initial masses of 15, 20, and 25M_⊙at solar metallicity. We calculated a new set of 24 CCSN models (eight for each stellar progenitor mass) and compared our results with another recently published set of 80 CCSN models that includes a wide range of explosion parameters: explosion energy or initial shock velocity, energy injection time, and mass location of the injection. Results.We find that the totalp-nucleus yields are only marginally affected by the CCSN explosion prescriptions if theγ-process production is already efficient in the stellar progenitors due to a C−O shell merger. In most CCSN explosions from progenitors without a C−O shell merger, theγ-process yields increase with the explosion energy by up to an order of magnitude, depending on the progenitor structure and the CCSN prescriptions. The general trend of thep-nucleus production with the explosion energy is more complicated if we look at the production of singlepnuclei. The lightp-nuclei tend to be the most enhanced with increasing explosion energy. In particular, for the CCSN models where theα-rich freeze-out component is ejected, the yields of the lightestpnuclei (including^92, 94Mo and⁹⁶Ru) increase by up to three orders of magnitude. Conclusions.We provide the first extensive study using different sets of massive stars of the impact of varying CCSN explosion prescriptions on the production ofpnuclei. Unlike previous expectations and recent results in the literature, we find that the average production ofpnuclei tends to increase with the explosion energy. We also confirm that the pre-explosion production ofpnuclei in C−O shell mergers is a robust result, independent of the subsequent explosive nucleosynthesis. More generally, a realistic range of variations in the evolution of stellar progenitors and in the CCSN explosions might boost the CCSN contribution to the galactic chemical evolution ofpnuclei. 

Context. The γ -process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average yields of γ -process nucleosynthesis from massive stars are still insufficient to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are a factor of ten lower than the yields of the other γ -process nuclei. Aims. We investigate the γ -process in five sets of core-collapse supernova models published in the literature with initial masses of 15, 20, and 25 M ⊙ at solar metallicity. Methods. We compared the γ -process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass relative to their solar values. Further, we investigated the contribution of C–O shell mergers in the supernova progenitors as an additional site of the γ -process. Results. Our analysis shows that a large scatter among the different models exists for both the γ -process integrated yields and the isotopic ratios. We find only ten ratios that agree with their solar values, all the others differ by at least a factor of three from the solar values in all the considered sets of models. The γ -process within C–O shell mergers mostly influences the isotopic ratios that involve intermediate and heavy proton-rich isotopes with A  > 100. Conclusions. We conclude that there are large discrepancies both among the different data sets and between the model predictions and the solar abundance distribution. More calculations are needed; particularly updating the nuclear network, because the majority of the models considered in this work do not use the latest reaction rates for the γ -process nucleosynthesis. Moreover, the role of C–O shell mergers requires further investigation.