A comparison of Rosseland-mean opacities from OP and OPAL

Abstract

Monochromatic opacities from the Opacity Project (OP) (Seaton et al.) have been augmented by hitherto missing inner-shell contributions (Badnell & Seaton). OP Rosseland-mean opacities, �R, are compared with results from OPAL (Iglesias & Rogers) for the elements H, He, C, O, S and Fe. The OPAL data are obtained from the website www-phys.llnl.gov/Research/OPAL/index.html. Agreement for H is close everywhere except for the region of log(T) ≃ 6 and log(R) ≃ −1 (R = �/T 3 6 whereis mass-density in g cm 3 and T6 = 10 6 ×T with T in K). In that regionR(OPAL) is larger thanR(OP) by up to 13%. The differences are due to different equations of state (EOS). In the region concerned OP has the H ground state undergoing dissolution, leading to a small H-neutral ionization fraction, while OPAL has larger values for that fraction. A similar difference occurs for He at log(R) ≃ −1 and log(T) ≃ 6.4, where OP has the He + ground-state undergoing dissolution. The OPAL website does not provide single-element Rosseland means for elements other than H and He. Comparisons between OP and OPAL are made for mixtures with X = 0.9, Z = 0.1 and Z containing pure C, O or S. There are some differences: at the lower temperatures, say log(T) 6 5.5, due to differences in atomic data, with the OP R-matrix data probably being the more accurate; and at higher temperatures mainly due to differences in level populations resulting from the use of different EOS theories. In the original OP work, R-matrix data for iron were supplemented by data ob- tained using the configuration-interaction (CI) code superstructure. The experi- ment is made of replacing much of the original iron data with new data from the CI code autostructure. Inclusion of intercombination lines gives an increase inR of up to 18%. The OPAL website does not allow for Z containing pure iron. Comparisons are made for an iron-rich mixture, X = 0.9, Z = 0.1 and Z containing C and Fe with C:Fe=2:1 by number fraction. There are some differences between OP and OPAL for that case: the OP 'Z-bump' inR is shifted to slightly higher temperatures, compared to OPAL. Overall, there is good agreement between OP and OPAL Rosseland-mean opacities for the 6-elements, but there are some differences. Recent work (Bahcall et al.) has shown that helioseismology measurements give a very accurate value for the depth of the solar convection zone, RCZ, and that solar models give agreement with that value only if opacities at RCZ are about 7% larger than OPAL values. For the 6-element mix at RCZ we obtainR(OP) to be larger thanR(OPAL) by 5%.