Rotational mixing in early-type stars: the main-sequence evolution of a 9 Mo star

Abstract

We describe the main-sequence evolution of a rotating 9 $M_\odot$ star. Its interior rotation profile is determined by the redistribution of angular momentum through the meridian circulation and through the shear turbulence generated by the differential rotation; the possible effect of internal waves is neglected. We examine the mixing of chemicals produced by the same internal motions. Our modelization is based on the set of equations established by Zahn (1992) and completed in Matias, Talon & Zahn (1996). Our calculations show that the amount of mixing associated with a typical rotation velocity of $\sim 100 km/s$ yields stellar models whose global parameters are very similar to those obtained with the moderate overshooting ($d/H_P \simeq 0.2$) which has been invoked until now to fit the observations. Fast rotation ($\sim 300 km/s$) leads to significant changes of the C/N and O/N surface ratios, but the abundance of He is barely increased. The modifications of the internal composition profile due to such rotational mixing will certainly affect the post--main-sequence evolution.