Production of radioactive $^{22}$Na in core-collapse supernovae: the Ne-E(L) component in presolar grains and its possible consequences on supernova observations

Abstract

Presolar graphite grains carry the isotopic signatures of their parent stars. A significant fraction of presolar graphites shows 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 the $^{22}$Ne-enriched component Ne-E(L), interpreted as the effect of the radioactive decay of $^{22}$Na ($T_{1/2}$ = 2.6 years). 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 $^{22}$Na amounts. Here we focus on such CCSN models, showing a radioactive $^{26}$Al production compatible with grains measurements, and analyze the conditions of $^{22}$Na nucleosynthesis. In these models, $^{22}$Na is mostly made in the He shell, with a total ejected mass varying between 2.6$\times$10$^{-3}$ M$_{\odot}$ and 1.9$\times$10$^{-6}$ M$_{\odot}$. We show that such $^{22}$Na 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 before the $^{44}$Ti decay becomes dominant. Based on the CCSN yields above, the 1274.53 keV $\gamma$-ray flux due to $^{22}$Na decay could be observable for years after the first CCSN light is detected, depending on the distance. This makes CCSNe possible sites to detect a $^{22}$Na $\gamma$-ray signature consistently with the Ne-E(L) component found in presolar graphites. Finally, we discuss the potential contribution from the $^{22}$Na decay to the galactic positron annihilation rate.