The first stars: a classification of CEMP-no stars

TL;DR

This paper introduces a new classification scheme for CEMP-no stars based on element and isotope abundance patterns resulting from nuclear cycles in massive source stars, shedding light on early Galactic chemical evolution.

Contribution

A novel classification method for CEMP-no stars based on detailed abundance ratios linked to nucleosynthesis processes in massive stars.

Findings

Abundance ratios vary with [Fe/H], indicating different nucleosynthetic histories.

The classification supports the role of fast-rotating massive stars in early Galaxy evolution.

[(C+N+O)/Fe] ratios increase as [Fe/H] decreases.

Abstract

We propose and apply a new classification for the CEMP-no stars, which are "carbon-enhanced metal-poor" stars with no overabundance of s-elements and with [Fe/H] generally inferior or equal to -2.5. This classification is based on the changes in abundances for the elements and isotopes involved in the CNO, Ne-Na, and Mg-Al 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 CEMP-no stars). 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 CEMP-no 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]. 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.