STELLAR EVOLUTION WITH ARBITRARY ROTATION LAWS .2. MASSIVE STAR EVOLUTION TO CORE HYDROGEN EXHAUSTION

Core convection has been added to a fully implicit two-dimensional hydrostatic and hydrodynamic code developed to study (among other problems) stellar evolution with arbitrary rotation laws. This code allows the calculation of both the evolution associated with nuclear burning and the evolution associated with the redistribution of angular momentum generated by a limited range of instabilities. The inclusion of core convection was performed in a rather unorthodox manner in order to maintain our ability to examine a wide spectrum of hydrodynamic and quasi-static problems. Other changes in our method include performing the composition changes due to nuclear processing and advection through the non-Lagrangian mesh implicitly rather than explicitly. Another important problem addressed is the evolution of the core angular momentum. We calculate a nonrotating and two rotating evolutionary sequences of an 8.75 solar mass model from the zero-age main sequence to the end of core hydrogen burning. In the rotating models the convective core is assumed to rotate as a solid body. Unlike the usual one-dimensional Lagrangian models, we must calculate the small velocities produced by the long-timescale evolution of the model in order for the mass, composition, and angular momentum to move properly with respect to the non-Lagrangian grid. These evolutionary velocities include a nonspherical component in the rotating models. Approximately midway through core hydrogen burning a dynamical instability is encountered near the outer boundary of the convective core in the rotating models. This is followed on the appropriate hydrodynamic timescale, producing velocities of tens of meters per second when the instability is initially encountered, but increasing to a few kilometers per second by the end of core hydrogen burning. These velocities generate a net transport of angular momentum from the convective core to the regions just exterior to the core and simultaneously increase the hydrogen abundance in the convective region. Both the observable evolution and the central properties are insignificantly altered in these models by this process.