The SPAr burning: proton captures powering carbon-oxygen shell mergers in massive stars

Abstract

Carbon-oxygen (C-O) shell mergers in massive stars play a crucial role in both nucleosynthesis and the final stages of stellar evolution. These convective-reactive events significantly alter the internal structure of the star shortly before core collapse. We investigate how the enhanced production of light particles (especially protons) during a C-O shell merger, relative to classical oxygen shell burning, affects the energy balance and evolution of the convective shell. We derive the budget for direct and reverse nucleosynthesis flows across all relevant nuclear reactions from stellar evolution models, and we assess the relative energy produced. We find that proton capture reactions on 32,34S, 31P, and 38Ar (SPAr) dominate the nuclear energy production in typical C-O shell mergers as predicted by 1D stellar models. Their combined energy output is approximately 400 times greater than that of C and O fusion under the same conditions. Our results highlight the critical importance of including these proton-capture reactions in simulations of convective-reactive burning. This work suggests that excluding their contribution can lead to inaccurate modeling of the dynamics and nucleosynthesis in advanced stellar evolutionary phases. Such results will need to be confirmed by new 1D stellar simulations and 3D hydrodynamics models.

Figures (2)

• Figure 1: The relative contribution of individual effective nuclear reactions (forward minus reverse reaction, see text) to the total nuclear energy production at the bottom of the O-burning shell during O shell burning (upper panel), at the beginning of the C--O shell merger (central panel), and at the end of the C--O shell merger (lower panel). The blue bars represent processes that are dominated by the forward reaction and have a positive energy contribution, while red bars represent instead processes that are dominated by the reverse reaction and have a negative energy contribution. The three panels include all the reactions having an absolute $\varepsilon/\varepsilon_{tot}>10^{-3}$ (See also \ref{['tab:eratio1']}-\ref{['tab:eratio2']}-\ref{['tab:eratio3']}).
• Figure 2: Abundances in mass fraction as a function of the internal mass coordinate for key nuclear species (see legend). The grey shaded area represents the O convective shell $\rm 3.47\times10^{5}\ s$ (upper panel), $\rm 6.66\times10^{3}\ s$ (central panel) and $\rm 5.72\times10^{2}\ s$ (lower panel) before the core-collapse. Neutron and proton abundances (not shown in the figure) with other relevant species are provided at these different stages in Table \ref{['tab:oburn']}, at the base of the grey zone. The dotted-dashed red line shows the cumulative integral of the energy per second generated by the nuclear reactions from the outer edge to the bottom of the O shell normalized to its maximum.