The first stars: A classification of CEMP-no stars

Abstract

Aims. We propose and apply a new classification for the CEMPno stars, which are carbon-enhanced metal-poor stars with no overabundance of s-elements and with [Fe/H] generally 2.5. Methods. This classification is based on the changes in abundances for the elements and isotopes involved in the CNO, NeNa, and MgAl nuclear cycles. These abundances change very much owing to successive back and forth mixing motions between the He- and H-burning regions in massive stars (the "source stars" responsible for the chemical enrichment of the CEMPno stars). Results. The wide variety of the ratios [C/Fe], 12C/13C, [N/Fe], [O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe], [Sr/Fe], and [Ba/Fe], which are the main characteristics making the CEMPno and low-s stars so peculiar, is described well in terms of the proposed nucleosynthetic classification. We note that the [(C+N+O)/Fe] ratios significantly increase for lower values of [Fe/H]. Conclusions. The classification of CEMP-no stars and the behavior of [(C+N+O)/Fe] support the presence, in the first stellar generations of the Galaxy, of fast-rotating massive stars experiencing strong mixing and mass loss (spinstars). This result has an impact on the early chemical and spectral evolution of the Galaxy.