The occurrence and impact of 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 Miklós út 15-17, Budapest, H-1121, Hungary
³ CSFK HUN-REN, MTA Centre of Excellence, Konkoly Thege Miklós út 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 E. A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston upon Hull, HU6 7RX, UK
⁷ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium
⁸ Department of Physics, Indian Institute of Technology Palakkad, Kerala, India
⁹ Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA, 94550, USA
¹⁰ Dipartimento di Fisica, Sapienza Università di Roma, P.le A. Moro 5, Roma, I-00185, Italy
¹¹ Kavli Institute for the Physics and Mathematics of the Universe, Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, 277-8583 (Kavli IPMU, WPI), Japan
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Via A. Pascoli s/n, Perugia, I-06125, Italy
¹³ Istituto Nazionale di Astrofisica–Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, Roma, I-00133, Italy
¹⁴ School of Physics and Astronomy, Monash University, VIC 3800, Australia
¹⁵ ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Budapest 1117, Pázmány Péter sétány 1/A, Hungary

^⋆ Corresponding author.

Received: 10 March 2025
Accepted: 24 April 2025

Abstract

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-Z elements 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 the ¹²C 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_C12 and M_CO are 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-Z elements. This highlights the necessity of further investigations to refine their influence on presupernova stellar properties and their broader impact on Galactic chemical evolution.