\begin{center}{\Large\bf 
Effect of Partial Mixing of Matter on the Hydrodynamic Angular Momentum Transport Processes in Massive Main-Sequence Stars.}\end{center} 

\centerline{\bf E.I. Staritsin
}{\footnotesize Astronomical Observatory, Ural State University, Yekaterinburg, Russia
}
 
We consider the evolution of a rotating star with a mass of $16 M_\odot$ and an angular momentum of $3.25\times10^{52} g\cdot cm^2c^{-1}$, along with the hydrodynamical transport of angular momentum and chemical elements in its interiors. When the partial mixing of matter of the turbulent radiative envelope and the convective core is taken into account, the efficiency of the angular momentum transport by meridional circulation in the stellar interiors and the duration of the hydrogen burning phase increase. Depending on the Schmidt number in the turbulent radiative stellar envelope, the ratio of the equatorial rotational velocity to circular one increases with time in the process of stellar evolution and can become typical of early-type Be stars during an additional evolution time of the star on the main sequence. Partial mixing of matter is a necessary condition under which the hydrodynamic transport processes can increase the angular momentum of the outer stellar layer to an extent that the equatorial rotational velocity begins to increase during the second half of the evolutionary phase of the star on the main sequence, as shown by observations of the brightest stars in open star clusters with ages of 10--25 Myr. When the turbulent Schmidt number is 0.4, the equatorial rotational velocity of the star increases during the second half of the hydrogen burning phase in the convective core from 330 to 450 $km\,s^{-1}$.

{\bf Reference: Astronomy Letters} \\{\it Status:} Manuscript has been accepted
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{\it On the web at:} {\tt } 

{\it Preprints from:} {\tt  Eugenij.Staritsin@usu.ru} 
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