| Abstract
| - In stars which are sufficiently quiescent, changes in the relative abundances of the chemical elements can result from gravitational settling and from lévitation produced by radiation pressure forces, usually expressed as radiative accelerations grad. Those changes can affect the structure of such stars, due to modifications in opacities, and can lead to marked peculiarities in observed atmospheric abundances. It is necessary to consider diffusive movements both in the atmospheres and in much deeper layers of the stellar envelopes. For the envelopes the equation of radiative transfer can be solved in a diffusion approximation and, for an elementk in ionization stage j, one obtains expressions for grad (j,k) proportional to the total radiative flux, to the Rosseland-mean opacity kr (which may depend on the abundance of k), and to a dimensionless quantity γ(j, k) which, due to saturation effects, can be sensitive to the abundance of k. The radiative accelerations are required for each ionization stage, because the diffusion coefficients depend on j. Using atomic data obtained in the course of the work of the Opacity Project (OP), we calculate kr and γ (j, k) for the chemical elements C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. We start from standard Solar system abundances, and then vary the abundance of one element at a time (element k) by a factor x The following results are obtained and are available at the Centre de Données astronomiques de Strasbourg (CDS). (1) Files stages.zz (where zz specifies the nuclear charge of the selected element k) containing values of kr and γ(j,k) on a mesh of values of (T, Ne, ϰ), where T is temperature, andNe is electron density. We include derivatives of kr and γ (j, k) with respect to ϰ, which are used for making interpolations. (2) A code add. f which reads a file stages.zz and writes a file acc.zz containing values of γ (k) obtained on summing the γ (j, k), weighted by diffusion coefficients. The diffusion coefficients to be employed can be selected by the user. (3) A code acc . f which reads a file acc.zz and provides facilities for interpolations of kr and gad(fc) to values of (T, ρ, ϰ) for a stellar model, where ρ is mass density. The mesh to be used for log(ϰ) is specified by the user. (4) A code dif f . f intended for use in diffusion calculations. It reads a file created by acc . f and provides function subroutines for the calculation of kr and grad (k) for any specified depth-point and any value of ϰ Results are compared with those from other recent work for C, N, O, Ca and Fe.
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