Chemical evolution of close massive binaries – Tidally enhanced or tidally suppressed mixing

Department of Astronomy, University of Geneva Chemin Pegasi 51 CH-1290 Versoix, Switzerland

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Received: 26 August 2025
Accepted: 19 January 2026

Abstract

Context. One of the largest sources of uncertainty in the predictions of stellar models comes from the internal transport mechanisms. In close massive binaries, previous theoretical studies suggest that tides consistently boost chemical mixing. However, observations do not reveal any clear period-nitrogen enrichment trend, challenging these predictions. In addition, comprehensive examinations of the interplay between tidal interactions, angular momentum, and chemicals transport have so far been very scarce.

Aims. Our goal is to investigate the interplay between tidal interactions and rotational mixing, and the impact of the angular moment transport (AMT) assumptions. We also aim to tackle the question of whether tidal interactions enhance or suppress chemical mixing.

Methods. We computed grids of GENEC binary models with various AMT treatments at solar metallicity. In order to independently assess the role of tidal interactions, we systematically computed model variations of single stars with identical initial conditions.

Results. Our investigations reveal that tidal interactions can either enhance or suppress mixing relative to single-star models with identical initial conditions, and that the outcome is highly sensitive to the adopted AMT assumptions. We identify a key contrast between the two types of computed models: in close systems subject to tides, magnetic models predict that the mixing efficiency is mostly determined by the orbital configuration, whereas in hydrodynamic models it also depends on the assumed initial velocity. As a result, hydro models may display non-monotonic period–enrichment trends, or even period-enrichment correlations.

Conclusions. These results highlight the importance of the AMT assumptions in modeling binaries with tidal interactions, notably in the context of the chemically homogeneous evolution channel. The sensitivity of the predictions of hydro models to initial conditions extends the size of the period-enrichment parameter space they cover, allowing them to accommodate for peculiar observed systems, i.e., with mild enrichment at short periods or high enrichment at longer periods.