Envelope

Abstract

We present radiative Rosseland mean opacity tables calculated with the OPAL code (Iglesias and Rogers 1991) which removes several approximations present in previous calculations. In particular, improvements in the atomic physics have been incorporated which yield reasonably accurate photon absorption data (accuracy comparable to single-configuration, self-consistent field calculations with relativistic corrections) and include the configuration term splitting in the LS-coupling scheme. In some cases, the latter can increase the number of spectral lines by orders of magnitude compared to previous opacity codes. OPAL also employs an equation of state method which avoids the usual ad hoc cut offs introduced in free energy minimizaton schemes. The opacity tables are given in terms of temperature and R, where R = p/T^ with p the density and T the temperature. Extensive results are given for the Anders-Grevesse (1989) metal abundances which allow accurate interpolation in temperature, density, hydrogen mass fraction, and metal mass fraction. The range of T and R cover typical stellar conditions from the Beat Cepheids are classical Cepheids which pulsate simultaneously in the fundamental and first overtone. The two periods can be accurately disentangled and extracted from observations. In attempting to analyze such stars, use is made of the Petersen diagram where the ratio of the first overtone to fundamental period, Pl/P0,is plotted against the fundamental period P0. One approach in modelling such stars is to use linear non-adiabatic theory, with masses and luminosities mandated by stellar evolutionary calculations, to compute the fundamental and first overtone modes. The spectra of highly charged ions of NEON, ARGON, KRYPTON and XENON with few valence electrons have been studied in the 300-2100 A range using a pulsed discharge tube. For Ne spectra we found lines up to the sixth degree of ionization, We have developed a powerful technique for compacting and assessing atomic data for electron impact excitation of positive ions. Rate coefficients for such processes are derived from thermally average collision strengths T(T) which usually vary smoothly with temperature. The present method begins by scaling T in order to produce a reduced form T r . This is then plotted as a function of the scaled temperature T r which maps the entire range of T onto the interval (0,1). Y r can be accurately fitted by a 5-point cubic spline, so economising on storage while at the same time providing a convenient means for interpolating and extrapolating data. Straightforward tabulation normally requires more data points than this to cover a limited temperature range. With increasing amounts of atomic data becoming available, the problem of storage needs careful consideration. The present method is a possible solution. It forms the basis of an interactive computer program called OMEUPS which uses graphical display and is designed to be convenient for use by astrophysicists as well of those working in atomic collision theory. Some graphic examples will be presented to illustrate the method. The precision and accuracy of modern high resolution spectroscopic observations of cosmic bodies continues to drive the need for improved laboratory data and theories of atoms and molecules. Inparticular, recent measurements of asymmetric molecular hydrogen quadrupole lines in the absorption spectrum of Uranus and observations of high / rotational transitions in the infrared emission spectrum of molecular hydrogen in the Orion nebulae point to the need for high resolution and high accuracy data. We have recently completed the measurement of pressure shifts, line strengths and shapes, broadening coefficients and line positions for the S(0) and S(l) lines of molecular hydrogen in the 4-0 vibration-rotation band. We have also detected for the first time in the laboratory the 5-0 S(l) line and confirm its detection in Uranus. With these and other high quality measurements of the overtone spectrum of molecular hydrogen in the infrared and visible, a consistent set of parameters has emerged which should be useful to astrophysicists and may challenge theorists to improve their ab-initio calculations. Approximations in Brueckner's theory of spectral broadening by collisions with neutral hydrogen atoms relevant to a solar-type atmosphere have been discussed, and a modified theory for s-p transitions has been presented. The theory utilises explicit expressions for the interatomic interaction energy between a hydrogen atom in its ground state and general m = 0, ± 1 p-states, derived from second-order perturbation theory without exchange, allowing for removal of the Lindholm-Foley average over m-states in the original Brueckner model. Approximate upper and lower bounds for the linewidth of the sodium D-lines are derived, and these values are contrasted with available theoretical, experimental and solar empirical results. The removal of the Lindholm-Foley average is shown to reduce the D-state linewidths by about 30%, and an analysis of the interatomic separations important in the line-broadening cross-section for the D-lines has shown that there is little atomic overlap at the separations that are important.