Mixing due to internal gravity waves can explain the CNO surface abundances of B-type detached eclipsing binaries and single stars

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 June 2025
Accepted: 22 July 2025

Abstract

Context. Observations of double-lined spectroscopic eclipsing binaries are ideal for studies of stellar evolution. Such stars have tight model-independent constraints on their masses and radii. When used in combination with spectroscopically determined effective temperatures and surface abundances, they can be used to calibrate and improve stellar evolution models.

Aims. The main goal of this work is to determine whether the observed trends of surface nitrogen abundance in single and binary stars can be explained by wave-induced mixing occurring in the stellar envelope.

Methods. We used the MESA stellar evolution code to run simulations of single B-type stars with envelope mixing induced by internal gravity waves. We compared the outcome of these models to observations of the surface nitrogen abundance for samples of detached eclipsing binary systems and of single B-type stars. From this comparison, we were able to determine the amount of wave-induced mixing required to bring the model predictions in agreement with the observations.

Results. We find nitrogen to be enriched at the surface of theoretical models with wave-induced mixing provided that we use levels above log(D_env/cm² s⁻¹) = 5–6 at the position of the convective core boundary. This corresponds to the highest levels of envelope mixing derived from asteroseismic modelling of single B stars. A prominent observation is that the B-type components of detached eclipsing binaries do not show any nitrogen surface enhancement, which can be explained by their relatively fast rotation enforced by the tidal forces in the systems. The slowly rotating or evolved stars among the sample of single B stars do reveal a nitrogen enhancement. Our findings on the difference in surface nitrogen abundances between single B stars and B-type components of detached binary systems could potentially be explained by internal wave-induced mixing profiles based on recent two-dimensional hydrodynamical simulations of rotating B stars with waves excited at the interface between the convective core and radiative envelope. Such wave-induced mixing decreases with increasing rotation and might act in combination with additional rotational mixing.

Conclusions. Our findings motivate future asteroseismic studies in large samples of single B stars and pulsating eclipsing binaries with B-type components as optimal laboratories to further test our interpretations in terms of internal wave mixing.