Explaining the

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 <ns0:monospace>GEMS</ns0:monospace> 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.