Mixing of CNO-cycled matter in pulsationally and magnetically active massive stars

TL;DR

This study analyzes the chemical abundances in massive Galactic stars, revealing a link between magnetic fields and internal mixing, with implications for stellar evolution and magnetic braking.

Contribution

It provides the first homogeneous analysis of CNO abundances in a diverse sample of massive stars, highlighting the role of magnetic fields in stellar mixing processes.

Findings

Stars follow a tight N/C-N/O trend consistent with CNO cycle predictions.

Magnetic stars show enhanced mixing signatures compared to non-magnetic stars.

Magnetic braking appears highly efficient in spinning down some massive stars.

Abstract

We report on the abundances of helium, carbon, nitrogen and oxygen in a larger sample of Galactic massive stars of ~7-20 M_sun near the main sequence, composed of apparently normal objects, pulsators of beta-Cephei- and SPB-type, and magnetic stars. High-quality spectra are homogeneously analysed using sophisticated non-LTE line-formation and comprehensive analysis strategies. All the stars follow a previously established tight trend in the N/C-N/O ratio and show normal helium abundances, tracing the nuclear path of the CNO-cycles quantitatively. A correlation of the strength of the mixing signature with the presence of magnetic fields is found. In conjunction with low rotation velocities this implies that magnetic breaking is highly efficient for the spin-down of some massive stars. We suggest several objects for follow-up spectropolarimetry, as the mixing signature indicates a possible magnetic nature of these stars.