ASTROPHYSICS Main - sequence stars of 10 and 30 M : approaching the steady - state rotation

The evolution of the angular velocity profile in 10 and 30M main-sequence (MS) stars has been calculated from the initial uniform rotation to the asymptotic steady-state rotation. Following Zahn (1992) we assume that both transport of angular momentum and mixing of chemical elements are produced by rotationally induced meridional circulation and turbulent diffusion. It is shown that for a sufficiently large surface rotational velocity, whose value can be estimated a priori, the relaxation time for star’s achieving the steady-state rotation is much shorter than the star’s MS life-time. In this case the assumption that a star is in a state of stationary rotation from the very beginning of its MS evolution is quite reasonable. On the other hand, for a star rotating slowly one has to solve the nonstationary angular momentum transport equation simultaneously with the stellar evolution calculations. Despite the fact that the rate of mixing of chemical elements by meridional circulation is strongly reduced by horizontal erosion, diffusion-like abundance profiles of C and N are built up in the radiative envelope by the end of the star’s MS life. The surface N abundance begins to increase after some delay time required for the diffusion wave to reach the atmosphere. If mixing penetrates the convective core the abundance of He is expected to behave like that of N. Internal gravity waves generated near the convective core border are shown to probably play an important role as another angular momentum transport mechanism, especially in the inner part of the envelope.