Letter to the Editor

The SPAr burning: Proton captures powering carbon–oxygen shell mergers in massive stars

¹ Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania I-95123, Italy
² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, Konkoly Thege Miklös üt 15-17, Budapest H-1121, Hungary
³ CSFK HUN-REN, MTA Centre of Excellence, Konkoly Thege Miklös üt 15-17, Budapest H-1121, Hungary
⁴ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone I-00040, Italy
⁵ NuGrid Collaboration, http://nugridstars.org
⁶ University of Bayreuth, BGI, Universitätsstraße 30, 95447 Bayreuth, Germany

^⋆ Corresponding author: lorenzo.roberti@csfk.org

Received: 22 August 2025
Accepted: 4 November 2025

Abstract

Context. Carbon–oxygen (C–O) shell mergers in massive stars play a crucial role in nucleosynthesis and in the final stages of stellar evolution. These convective-reactive events significantly alter the internal structure of the star shortly before core collapse.

Aims. We investigated 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.

Methods. We derived the budget for direct and reverse nucleosynthesis flows across all relevant nuclear reactions from stellar evolution models, and we assessed the relative energy produced.

Results. 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.

Conclusions. 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. These results will need to be confirmed by new 1D stellar simulations and 3D hydrodynamics models.