Detailed Opacity Calculations for Astrophysical Applications

Abstract

Nowadays, several opacity codes are able to provide data for stellar structure models, but the computed opacities may show significant differences. In this work, we present state-of-the-art precise spectral opacity calculations, illustrated by stellar applications. The essential role of laboratory experiments to check the quality of the computed data is underlined. We review some X-ray and XUV laser and Z-pinch photo-absorption measurements as well as X-ray emission spectroscopy experiments involving hot dense plasmas produced by ultra-high-intensity laser irradiation. The measured spectra are systematically compared with the fine-structure opacity code SCO-RCG. The focus is on iron, due to its crucial role in understanding asteroseismic observations of β Cephei-type and Slowly Pulsating B stars, as well as of the Sun. For instance, in β Cephei-type stars, the iron-group opacity peak excites acoustic modes through the "kappa-mechanism". Particular attention is paid to the higher-than-predicted iron opacity measured at the Sandia Z-machine at solar interior conditions. We discuss some theoretical aspects such as density effects, photo-ionization, autoionization or the "filling-the-gap" effect of highly excited states.

Combining multiple structural inversions to constrain the solar modelling problem

“…The main difficulty with this issue is that the observed discrepancies can originate from multiple sources. Amongst them, the alleged largest contributor is the radiative opacity, which is thought to be underestimated for the physical conditions of the BCZ (Bailey et al 2015;Iglesias 2015;Nahar & Pradhan 2016;Zhao et al 2018;Pain et al 2018), although the origin of the discrepancy is still under debate (Blancard et al 2016;Iglesias & Hansen 2017;Pain et al 2017). However, other inputs of standard models can also contribute to the discrepancies, such as the equation of state, the formalism used for microscopic diffusion, the formalism of convection or the nuclear reaction rates.…”

Solar structure and evolution

“…The computation of stellar opacities is generally so complicated that opacities have to be obtained in stellar modelling through interpolation in tables. The computation of the tables includes contributions of transitions between the different levels of the atoms and ions in the gas, including as far as possible the effects of level perturbations; an extensive review of opacity calculations was provided by Pain et al (2017). The thermodynamic state of the gas, including the degrees of ionization and the distribution amongst the levels, is an important ingredient in the calculation; indeed, both the MHD and the OPAL equations of state were developed as bases for new opacity calculations.…”

Photoionization of Astrophysically Relevant Atomic Ions at PIPE

“…The interpretation of the astronomical observations of cosmic plasmas requires a quantitative understanding of the quantum processes that lead to the emission or absorption of photons and that govern the charge balance of atoms and ions in a plasma. The photoionization and photoabsorption of atomic ions are important in connection with radiation transport, e.g., in stars [1] or kilonovae [2] and whenever a cosmic plasma is within the line of sight between the observer and a radiation source such as, e.g., a star, an X-ray binary, or an active galactic nucleus. Absorption spectra that were recorded by X-ray telescopes contain spectral signatures of the atomic ions contained in the plasma [3].…”