A treatment of opacity suitable for media of low density and temperature. I

The paper is concerned with the determination of mean absorption coefficients for the most important physical processes, suitable for a wide range of physical conditions. In particular, they do not need any particularly restrictive assumption about the thermodynamical state of matter, and about the coupling between matter and radiation. All these reasons, and the choice of the Planck mean, make the authors' mean absorption coefficients especially useful in those particular low density and temperature conditions, that are typical of protogalaxies and protostars. Furthermore, the opacities are given in an explicit functional form which allows to avoid interpolation of large multi-parameter tables.