Mixing of CNO-cycled matter in massive stars

TL;DR

This study compares observed CNO abundance ratios and helium buildup in massive stars with theoretical models, confirming strong mixing processes and the catalytic nature of CNO cycles through detailed spectral analysis.

Contribution

It provides the first self-consistent observational evidence of the N/C vs. N/O trend and helium buildup across different stellar evolutionary stages, supporting models with strong mixing mechanisms.

Findings

Confirmed the catalytic nature of CNO cycles quantitatively.

Observed a tight N/C vs. N/O trend across stellar evolution stages.

Supported models with magnetic fields and dredge-up processes.

Abstract

Aims: We test predictions of evolution models on mixing of CNO-cycled products in massive stars from a fundamental perspective. Relative changes within the theoretical C:N:O abundance ratios and the buildup of helium are compared with observational results. Methods: A sample of well-studied Galactic massive stars is presented. High-quality optical spectra are carefully analysed using improved NLTE line-formation and comprehensive analysis strategies. The results are put in the context of the existing literature data. Results: A tight trend in the observed N/C vs. N/O ratios and the buildup of helium is found from the self-consistent analysis of main-sequence to supergiant stars for the first time. The catalytic nature of the CNO-cycles is confirmed quantitatively, though further investigations are required to derive a fully consistent picture. Our observational results support the case of strong mixing, as predicted e.g. by evolution models that consider magnetic fields or by models that have gone through the first dredge-up in the case of many supergiants.