RADIATIVE ACCELERATIONS IN STELLAR ENVELOPES

In stars which are sufficiently quiescent, changes in the relative abu ndances of the chemical elements can result from gravitational settlin g and from levitation produced by radiation pressure forces, usually e xpressed as radiative accelerations g(rad). Those changes can affect t he structure of such stars, due to modifications in opacities, and can lead to marked peculiarities in observed atmospheric abundances. It i s necessary to consider diffusive movements both in the atmospheres an d in much deeper layers of the stellar envelopes. For the envelopes th e equation of radiative transfer can be solved in a diffusion approxim ation and, for an element k in ionization stage j, one obtains express ions for g(rad) (j, k) proportional to the total radiative flux, to th e Rosseland-mean opacity kappa(R) (which may depend on the abundance o f k), and to a dimensionless quantity gamma (j, k) which, due to satur ation effects, can be sensitive to the abundance of k. The radiative a ccelerations are required for each ionization stage, because the diffu sion coefficients depend onj. Using atomic data obtained in the course of the work of the Opacity Project (OF), we calculate kappa(R) and ga mma (j, k) for the chemical elements C, N, O, Ne, Na, Mg, Al, Si, S, A r, Ca, Cr, Mn, Fe and Ni. We start from standard Solar system abundanc es, and then vary the abundance of one element at a time (element k) b y a factor chi. The following results are obtained and are available a t the Centre de Donnees astronomiques de Strasbourg (CDS). (1) Files s tages.zz (where zz specifies the nuclear charge of the selected elemen t k) containing values of kappa(R), and gamma (j, k) on a mesh of valu es of (T, N-e, chi), where T is temperature, and N-e is electron densi ty. We include derivatives of kappa(R) and gamma (j, k) with respect t o chi, which are used for making interpolations. (2) A code add.f whic h reads a file stages.zz and writes a file acc.zz containing values of gamma (k) obtained on summing the gamma (j, k), weighted by diffusion coefficients. The diffusion coefficients to be employed can be select ed by the user. (3) A code acc.f which reads a file acc.zz and provide s facilities for interpolations of kappa(R), and g(rad)(k) to values o f (T, rho, chi) for a stellar model, where p is mass density. The mesh to be used for log(chi) is specified by the user. (4) A code diff.f i ntended for use in diffusion calculations. It reads a file created by acc.f and provides function subroutines for the calculation of kappa(R ) and g(rad) (k) for any specified depth-point and any value of chi. R esults are compared with those from other recent work for C, N, O, Ca and Fe.