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

Abstract

Observations of double-lined spectroscopic eclipsing binaries are ideal to study stellar evolution. They have tight model-independent constraints on their masses and radii. With the addition of spectroscopically determined effective temperatures and surface abundances, they can be used to calibrate and improve models. Here we 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. We use MESA to run the simulations. We compare the outcome of the models to observations of the surface nitrogen abundance for samples of detached eclipsing binary systems and of single B-type stars. From this we determine the amount of wave-induced mixing required to bring the model predictions in agreement with the observations. We find nitrogen to be enriched at the surface of theoretical models with wave-induced mixing provided that we use levels above log(Denv)=5-6 at the convective core boundary. 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 between single B stars and B-type components of detached binary systems can potentially be explained by internal wave-induced mixing profiles based on recent 2-dimensional hydrodynamical simulations of rotating B stars. Such wave-induced mixing decreases with increasing rotation and may act in combination with additional rotational mixing. Our findings motivate future asteroseismic studies in samples of single B stars and pulsating eclipsing binaries with B-type components as optimal laboratories to further test our interpretations.