Rotational mixing in tidally locked massive main sequence binaries

Abstract One of the main uncertainties in evolutionary calculations of massive stars isthe efficiency of internal mixing. It changes the chemical profile inside the starand can therefore affect the structure and further evolution.We demonstrate that eclipsing binaries, in which the tides synchronize therotation period of the stars and the orbital period, constitute a potentiallystrong test for the efficiency of rotational mixing. We present detailed stellarevolutionary models of massive binaries assuming the composition of the SmallMagellanicCloud. In these models we find enhancements in the surface nitrogenabundance of up to 0.6 dex. Introduction The inclusionof rotation into stellarevolution models has been shown to be verysuccessful in explaining various observed characteristics of stars (see Maeder M Eddington 1925, 1926; Vogt 1925) and shear mixing, which re-sults from eddies formed between two layers of the star rotating at differentangular velocities.Near the center of a massive main sequence star hydrogen is converted intohelium, and carbon and oxygen into nitrogen. Rotational mixing can bring this