Explaining the ¹²C/¹³C ratio in the Galactic halo: The contribution from shell mergers in primordial massive stars

¹ Dipartimento di Fisica, Università degli Studi di Trieste, via Tiepolo 11, 34143 Trieste, Italy
² INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
³ INFN, Sezione di Trieste, via Valerio 2, 34134 Trieste, Italy
⁴ Institute for Fundamental Physics of the Universe, via Beirut, 2, 34151 Trieste, Italy
⁵ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
⁶ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
⁷ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁸ INAF/IAPS, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁹ INFN, Sezione di Perugia, via A. Pascoli s/n, 06125 Perugia, Italy
¹⁰ Monash Centre for Astrophysics (MoCA), School of Mathematical Sciences, Monash University, Victoria 3800, Australia
¹¹ Kavli IPMU (WPI), The University of Tokyo, Kashiwa, 277-8583 Chiba, Japan
¹² INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author: federico.rizzuti@inaf.it

Received: 24 December 2024
Accepted: 25 March 2025

Abstract

Context. Recent campaigns of observations have provided new measurements of the carbon isotopes in the most metal-poor stars of the Galaxy. These stars are so metal-poor that they could only have been enriched by one or few generations of massive progenitors. However, explaining the primary production of ¹³C and the low ¹²C/¹³C ratio measured in these stars is challenging.

Aims. Making use of the most up-to-date models for zero-metal and low-metallicity stars, we investigate possible sources of ¹³C at low metallicity and verify whether massive stars could be solely responsible for the ¹²C/¹³C ratio observed in halo stars.

Methods. We employed the stochastic model for Galactic chemical evolution GEMS to reproduce the evolution of CNO elements and ¹²C/¹³C ratio, including the enrichment from rotating massive stars, some of which show the occurrence of H-He shell mergers.

Results. We find that stars without H-He shell mergers do not produce enough ¹³C to be compatible with the observations. Instead, primary production via shell mergers and subsequent ejection during the supernova explosion can explain a ratio of 30 <¹²C/¹³C < 100. The observations are best reproduced assuming a large frequency of shell mergers. A ratio of ¹²C/¹³C < 30 can only be reproduced by assuming an outer layer ejection and no explosion, but requiring a higher level of production of ¹²C and ¹³C.

Conclusions. Zero-metal and low-metallicity spinstars with H-He shell mergers appear as the most plausible scenario to explain the low ¹²C/¹³C ratio in carbon-enhanced metal-poor (CEMP) stars. The entire range of ¹²C/¹³C values can be explained by assuming that some stars fully explode, while others only eject their outer layers. Shell mergers are also expected to be more frequent and productive, which is allowed by the current uncertainties related to the treatment of convection in stellar modelling.