Comparison of Fe and Ni opacity calculations for a better understanding of pulsating stellar envelopes

TL;DR

This paper compares Fe and Ni opacity calculations from eight different groups to improve understanding of stellar envelope pulsations and address discrepancies affecting stellar evolution models.

Contribution

It provides a comprehensive comparison of multiple opacity calculations for Fe and Ni under stellar conditions, highlighting differences and uncertainties.

Findings

Significant differences up to 50% in Fe radiative forces and Rosseland mean values.

Laboratory measurements of Ni and Fe spectra at relevant conditions.

Comparison reveals the need for improved opacity models for stellar physics.

Abstract

Opacity is an important ingredient of the evolution of stars. The calculation of opacity coefficients is complicated by the fact that the plasma contains partially ionized heavy ions that contribute to opacity dominated by H and He. Up to now, the astrophysical community has greatly benefited from the work of the contributions of Los Alamos [1], Livermore [2] and the Opacity Project (OP) [3]. However unexplained differences of up to 50% in the radiative forces and Rosseland mean values for Fe have been noticed for conditions corresponding to stellar envelopes. Such uncertainty has a real impact on the understanding of pulsating stellar envelopes, on the excitation of modes, and on the identification of the mode frequencies. Temperature and density conditions equivalent to those found in stars can now be produced in laboratory experiments for various atomic species. Recently the photo-absorption spectra of nickel and iron plasmas have been measured during the LULI 2010 campaign, for temperatures between 15 and 40 eV and densities of ~3 mg/cm3. A large theoretical collaboration, the "OPAC", has been formed to prepare these experiments. We present here the set of opacity calculations performed by eight different groups for conditions relevant to the LULI 2010 experiment and to astrophysical stellar envelope conditions.