Constraining mixing in massive stars in the Small Magellanic Cloud

Context. The evolution of massive stars is strongly influenced by internal mixing processes such as semiconvection, convective core overshooting, and rotationally induced mixing. None of these is currently well constrained. Aims. We investigate models for massive stars in the Small Magellanic Cloud (SMC). We aim to constrain the various mixing efficiencies by comparing model results to observations. Methods. We use the stellar evolution code MESA to compute more than 60 grids of detailed evolutionary models for stars with initial masses of 9 to 100 Msol, in each grid assuming different combinations of mixing efficiencies. Results. We find that for most of the combinations of the mixing efficiencies, models in a wide mass range spend core-helium burning either only as blue supergiants, or only as red supergiants. The latter case corresponds to models that maintain a shallow slope of the hydrogen/helium (H/He) gradient separating the core and the envelope of the models. Only a small part of the mixing parameter space leads to models that produce a significant number of blue and red supergiants, which both exist abundantly in the SMC. Interestingly, these models contain steep H/He gradients, as is required to understand the hot, hydrogen-rich Wolf-Rayet stars in the SMC. We find that unless it is very fast, rotation has a limited effect on the H/He profiles in our models. Conclusions. While we use specific implementations of the considered mixing processes, they comprehensively probe the two first order structural parameters: the core mass and the H/He gradient in the core-envelope interface. Our results imply that, in massive stars, the H/He gradients above the helium cores become very steep. Our model grids can be tested with future observational surveys of the massive stars in the SMC, and thereby help to considerably reduce the uncertainties in models of massive star evolution.