Description: |
The STAROX stellar evolution code. This paper describes the STAROX stellar evolution code for the calculation of the evolution of a model of a spherical star. The code calculates a model at time t k , that is the run of pressure, density, temperature, radius, energy flux and related variables on a mesh in mass M i , given the distribution of chemical elements X j ( i) at t k and the model at the previous time step t k-1. It then advances the chemical composition to the next time step t k+1 and calculates a new model at time t k+1. This process is iterated to convergence. The model equations are solved by Newton Raphson relaxation; the chemical equations are solved by an iterative procedure, each element being advanced in turn, and the process repeated to convergence. Convection is modelled by a mixing length model and convective mixing is treated as a diffusive process; chemical overshooting can be incorporated in parametric form. The equation of state is taken from OPAL tables and the opacity from a blend of OPAL and Alexander tables. Nuclear reaction rates are from NACRE but only cover the p p chain and CNO cycle. The atmospheric layers are incorporated in the model by applying the surface boundary condition at small optical depth ( τ≈0.001). The mesh in mass M i is usually taken as fixed except that there is a moveable mesh point at the boundary of a convective core. Results are given for models of mass 0.9 and 5.0 M ⊙ with initial composition X=0.7, Z=0.02 evolved to a state where the central hydrogen abundance is X c =0.35, and for a model of mass 2.0 M ⊙ with initial X=0.72, Z=0.02, evolved to X c =0.01 and with core overshooting. In this latter case we compute two models one with and one without a moveable mesh point at the boundary of the convective core to illustrate the importance of having such a moveable mesh point for the determination of the Brunt Väisälä frequency in the layers outside the core. |