OPACITIES FOR STELLAR ENVELOPES

We define stellar envelopes to be those regions of stellar interiors i n which atoms exist and are not markedly perturbed by the plasma envir onment. Availability of accurate and extensive atomic data is a prime requirement for the calculation of envelope opacities. For envelopes w e adopt the criterion of mass density rho less-than-or-equal-to 0.01 g cm-3. We present radiative Rosseland mean opacities for envelopes obt ained using atomic data calculated in an international collaboration r eferred to as the Opacity Project. or op. Equations of state are calcu lated using an occupation-probability formalism. To a good approximati on, ionization equilibria and level populations in envelopes depend on ly on the temperature T and electron density N(e) and are insensitive to chemical mixtures. Monochromatic opacities for all abundant chemica l elements are therefore calculated on a grid of (T, N(e)) values and are archived. Rosseland mean opacities are then readily calculated for any chemical mixture. Tables of Rosseland means, for any required mix tures and as functions of rho and T, are available on request in compu ter-readable form. The present, op, results are compared with those fr om another recent study, referred to as OPAL, by C. A. Iglesias and F. A. Rogers at the Lawrence Livermore National Laboratory. The agreemen t between the op and OPAL calculations is generally good, although the re are some differences. Both calculations give results larger than th ose obtained in earlier work, by factors of up to 3 or more.