The STAROX stellar evolution code

Abstract

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 tk, that is the run of pressure, density, temperature, radius, energy flux and related variables on a mesh in mass Mi, given the distribution of chemical elements Xj(i) at tk and the model at the previous time step tk−1. It then advances the chemical composition to the next time step tk+1 and calculates a new model at time tk+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 Mi 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 5.0M⊙ with initial composition X=0.7,Z=0.02 evolved to a state where the central hydrogen abundance is Xc=0.35, and for a model of mass 2.0M⊙ with initial X=0.72,Z=0.02, evolved to Xc=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–Vaisala frequency in the layers outside the core.