Forrest J. Rogers

UPDATED AND EXPANDED OPAL EQUATION-OF-STATE TABLES: IMPLICATIONS FOR HELIOSEISMOLOGY

We are in the process of updating and extending the OPAL equation-of-state (EOS) and opacity data to include low-mass stars. The EOS part of that effort now is complete, and the results are described herein. The new data cover main-sequence stars having mass � 0.1 M� . As a result of the more extreme matter conditions encountered with low-mass stars, we have added new physics. The electrons are now treated as relativistic, and we have improved our treatment of molecules. We also consider the implications of the new results for helioseismology.

Radiative atomic Rosseland mean opacity tables

For more than two decades the astrophysics community has depended on opacity tables produced at Los Alamos. In the present work we offer new radiative Rosseland mean opacity tables calculated with the OPAL code developed independently at LLNL. We give extensive results for the recent Anders-Grevesse mixture which allow accurate interpolation in temperature, density, hydrogen mass fraction, as well as metal mass fraction

Models for Old, Metal-poor Stars with Enhanced α-Element Abundances. I. Evolutionary Tracks and ZAHB Loci; Observational Constraints

Stellar evolutionary tracks have been computed for 17 [Fe/H] values from -2.31 to -0.30 assuming, in each case, [?/Fe] = 0.0, 0.3, and 0.6. The helium abundance was assumed to vary from Y = 0.2352 at [Fe/H] = -2.31 to Y = 0.2550 at [Fe/H] = -0.30 and held constant for the different choices of [?/Fe] at a fixed iron content. Masses in the range 0.5 ? ? ? 1.0, in 0.1 ? steps, were generally considered, though sequences for higher mass values were computed, as necessary, to ensure that isochrones as young as 8 Gyr could be generated for each grid. All of the stellar models are based on an equation of state that treats nonideal effects, the latest nuclear reaction and neutrino cooling rates, and opacities that were computed specifically for the adopted chemical mixtures. The tracks were extended to the tip of the giant branch or to an age of 30 Gyr, whichever came first, and zero-age horizontal-branch (ZAHB) loci were constructed using the helium core masses and chemical profiles from appropriate red giant precursors. Selected models have been compared with those computed by A. V. Sweigart, for the same masses and chemical compositions, to demonstrate that the results obtained from two entirely independent stellar evolution codes agree well with one another when very similar input physics is assumed. In the case of extremely metal-deficient stars, an enhancement in the abundance of the ?-elements causes a single, fairly significant bump in the opacity at a temperature just above 106 K, which is caused by absorption processes involving the K shell of oxygen. This peak becomes steadily more pronounced as the overall metallicity increases and a second bump, arising from the L edges of Ne, Mg, and Si, eventually appears near log T = 5.6. As far as the tracks and isochrones are concerned, we find that, as already reported by others, it is possible to mimic the computations for [?/Fe] > 0 remarkably well by those for scaled-solar mixes simply by requiring the total mass-fraction abundance of the heavy elements, Z, to be the same. However, this result holds only for metallicities significantly less than solar. Above [Fe/H] -0.8, tracks and isochrones for enhanced ?-element mixtures begin to have systematically hotter/bluer turnoffs and red giant branches than those for scaled-solar mixtures of the heavy elements. Also addressed is the extent to which our models satisfy the constraints posed by the local subdwarfs, the distances of which are based on Hipparcos parallax measurements. Our analysis suggests that the predicted metallicity dependence of the location of the lower main sequence on the C-M diagram is in good agreement with the observed dependence. In fact, we do not find any compelling evidence from the local Population II calibrators that the colors of our models require significant adjustments. In further support of our calculations, we find that, both in zero point and slope, the computed giant branches on the (Mbol, log Teff)-plane agree well with those inferred for globular clusters from observations in the infrared. Moreover, our ZAHB models have luminosities that are just outside the 1 ? error bars of the mean MVs inferred for RR Lyrae stars from Baade-Wesselink, statistical parallax, and trigonometric parallax studies. Lower helium contents or higher ?-element abundances or an increase in the conductive opacities are among the possible ways of reducing the differences that remain. To facilitate comparisons with observations, the tracks/ZAHBs are provided with predicted BV(RI)C photometry.

Spin-orbit interaction effects on the Rosseland mean opacity

Recently published opacity tables for astrophysical mixtures using the OPAL code showed significant differences when compared to the commonly used Los Alamos results. An important finding of the OPAL calculations was a large opacity enhancement (approximately factors of 3) in the few hundred-thousand degree temperature range. This new opacity bump is a consequence of improvements in the atomic data generation

A DRIVING MECHANISM FOR THE NEWLY DISCOVERED CLASS OF PULSATING SUBDWARF B STARS

We present new calculations that strongly reinforce the idea—originally proposed by Charpinet et al.—that pulsation modes are driven through an opacity bump due to a local enhancement of the iron abundance in the envelopes of sdB stars. Our improved models incorporate nonuniform iron abundance distributions obtained through the condition of diffusive equilibrium between gravitational settling and radiative levitation. They also include special Rosseland opacity tables that take into account the large variations of the iron abundance about the cosmic value that are predicted by equilibrium radiative levitation theory. For representative models with M = 0.48 M☉ and log g = 5.8, we find strong instabilities for low-order radial and nonradial (p and f) pulsation modes in the range 36,500 K Teff 29,000 K. The four pulsating sdB stars currently known all have effective temperatures in that range. In addition, one of our models with Teff = 34,000 K has a band of unstable modes with periods in the range 116-195 s, in excellent agreement with those of the known pulsators. We therefore claim that our proposed iron bump mechanism provides a natural explanation for the instabilities found in the newly discovered class of pulsating sdB stars.

Consistent Solar Evolution Model Including Diffusion and Radiative Acceleration Effects

The solar evolution has been calculated including all the effects of the diffusion of helium and heavy elements. Monochromatic opacities are used to calculate radiative accelerations and Rosseland opacities at each evolution time step, taking into account the local abundance changes of all important (21) chemical elements. The OPAL monochromatic data are used for the opacities and the radiative accelerations. The Opacity Project data are needed to calculate how chemical species and electrons share the momentum absorbed from the radiation flux. A detailed evaluation of the impact of atomic diffusion on solar models is presented. On some elements thermal diffusion adds approximately 50% to the gravitational settling velocity. While gravitational settling had been included in previous solar models, this is the first time that the impact of radiative accelerations is considered. Radiative accelerations can be up to 40% of gravity below the solar convection zone and thus affect chemical element diffusion significantly, contrary to current belief. Up to the solar age, the abundances of most metals change by 7.5% if complete ionization is assumed, but by 8.5%-10% if detailed ionization of each species is taken into account. If radiative accelerations are included, intermediate values are obtained. Diffusion leads to a change of up to 8% in the Rosseland opacities, compared to those of the original mixture. Most of this effect can be taken into account by using tables with several values of Z. If one isolates the effects of radiative accelerations, the abundance changes they cause alter the Rosseland opacity by up to 0.5%; the density is affected by up to 0.2%; the sound speed is affected by at most 0.06%. The inclusion of radiative accelerations leads to a reduction of 3% of neutrino fluxes measured with 37Cl detectors and 1% measured with 71Ga detectors. The partial transformation of C and O into N by nuclear reactions in the core causes a ~1% change in the opacities that cannot be modeled by a change in Z alone. The evolution is allowed to proceed to 1010 yr in order to determine the impact at the end of the main-sequence life of solar-type stars. It is found that immediately below the convection zone, the radiative acceleration on some iron peak elements is within a few percent of gravity. The abundance anomalies reach 18% for He in the convection zone but are kept within 12% and 15% for most because of grad. They would have reached 18% in the absence of grad.

Strongly coupled plasma physics

A NATO Advanced Research Workshop on Strongly Coupled Plasma Physics was held on the Santa Cruz Campus of the University of California, from August 4 through August 9, 1986. It was attended by 80 participants from 13 countries, 45 of whom were invited speakers. The present volume contains the texts of the invited talks and many of the contributed papers. The relative length of each text is roughly proportional to the length of the workshop presentation. The aim of the workshop was to bring together leading researchers from a number of related disciplines in which strong Coulomb interactions play a dominant role. Compared to the 1977 meeting in Orleans-la-Source, France and the 1982 meeting in Les-Houches, France, it is apparent that the field of strongly coupled plasmas has expanded greatly and has become a very significant field of physics with a wide range of applications. This workshop had a far greater participation of experimental researchers than did the previous two, and some confrontations of real experiments with theoretical calculations occurred. In the two earlier meetings the theoretical presentations were dominated by numerical simulations of static and dynamic properties of various strongly coupled plasmas. The dearth of experiments in the 1970s is now replaced by some very good experimental efforts.

Radiative opacities for carbon- and oxygen-rich mixtures

Recent OPAL calculations have obtained significant differences in the Rosseland mean opacities compared with earlier Los Alamos work. These new opacities have had a favorable impact on several astrophysical problems, but the efforts have concentrated on hydrogen main-sequence stars or stellar envelopes. The present calculations consider carbon- and oxygen-rich mixtures. It is shown that for such mixtures the Coulomb corrections beyond the weak-coupling approximation are not negligible in the ionization-balance calculations when Rosseland mean opacities are computed. As for hydrogen-rich compositions, the hydrogen-depleted mixtures can show factors of 2-3 enhancements in the opacity compared with the Los Alamos results at temperatures of a few hundred thousand degrees

Rosseland mean opacities for variable compositions

We have undertaken a long-term effort to provide improved Rosseland mean opacity tables. Our earlier work has shown that detailed atomic physics data is crucial to the accurate calculation of opacities. In particular, we showed that the inclusion of LS coupling in the configuration term structure can increase the opacity by factors of 3-4 over the Los Alamos results. Recently, we showed that the additional lines and the shifting of oscillator strength introduced by intermediate coupling can further increase the opacity by as much as 50%. In the present paper we present a method to expand our existing tables to include arbitrary values of hydrogen mass fraction

Radiative Accelerations for Evolutionary Model Calculations

Monochromatic opacities from the OPAL database have been used to calculate radiative accelerations for the 21 included chemical species. The 104 frequencies used are sufficient to calculate the radiative accelerations of many elements for T > 105 K, using frequency sampling. This temperature limit is higher for less abundant elements. As the abundances of Fe, He, or O are varied, the radiative acceleration of other elements changes, since abundant elements modify the frequency dependence of the radiative flux and the Rosseland opacity. Accurate radiative accelerations for a given element can only be obtained by allowing the abundances of the species that contribute most to the Rosseland opacity to vary during the evolution and recalculating the radiative accelerations and the Rosseland opacity during the evolution. There are physical phenomena that cannot be included in the calculations if one uses only the OPAL data. For instance, one should correct for the momentum given to the electron in a photoionization. Such effects are evaluated using atomic data from Opacity Project, and correction factors are given.