## 2. Basics of the modelingWe solve the four non-linear differential equations of the stellar structure by the method of Newton-Raphson, from center all the way to the surface, with the surface boundary conditions given as where is photospheric pressure, is the optical depth taken to be , and the remaining symbols have the usual meaning. Mass fraction of heavy elements is taken to be 0.019. Chemical abundances of the solar surface found by Anders & Grevesse (1989) are chosen. Nuclear reactions are from Caughlan & Fowler (1988). For convection, classical mixing length theory of Böhm-Vitense (1958) and Schwarzschild criteria are used. Stellar models are employed mostly in comparative works. Therefore the accuracy of the code is very important. For this reason the code is modified to obtain more accurate models. In order to acquire a higher accuracy, two points derivative of thermodynamic quantities are replaced with quadratic ones. The relative changes of the physical variables become smaller than for each shell boundaries in the solution of the equation of stellar structure by the Henyey method. © European Southern Observatory (ESO) 1997 Online publication: April 20, 1998 |