Atomic Physics at the Extreme: The Solar Abundance Problem

Dear Colleagues,

Over the past decade, a new solar problem has emerged, as solar photospheric abundances have been improved and the indicated solar metallicity, which is mainly due to low-Z metallic elements, has been significantly revised downward from that previously assumed. Standard solar models do not reproduce helioseismic observables when using these revised abundances. This has given rise to the solar composition problem. The amount of metals directly modifies the opacity profile of the Sun, and the solar composition problem is strongly related to the role of opacity in solar models. This has motivated a plethora of new studies, both experimental and theoretical, to understand atomic physics in the extreme environment of the Sun.

It is understood that the opacity of metals in the stellar mixture should be revised upward to compensate for the decreased abundances of low-Z metallic elements. Theoretically, it was found that details of the distribution of electrons and ions, which are highly uncertain, significantly affect the resulting opacity. In addition, the monochromatic opacity of iron, a major contributor to solar opacity, was recently measured in very similar conditions to those expected near the boundary of the solar convection zone. The measured spectrum appears to be larger than those calculated through various state-of-the-art and widely used atomic models by about a factor of two in several spectral regions near the L-shell photoabsorption lines. Moreover, measurements in neighbor elements at similar conditions, as well as in iron at different temperatures, do not show this effect. No satisfactory explanation for these observations has been provided.

This Special Issue of Atoms will highlight the need for continuing research on the different atomic properties affecting heat transport in the Sun as well as other main sequence stars and astrophysical phenomena. In addition, the importance of laboratory studies of hot and dense plasmas will be discussed. Particular emphasis will be put on new directions to study opacities in solar conditions and their relation to the solar abundance problem.

Prof. Dr. Doron Gazit
Guest Editor

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• Solar Physics
• Main sequence stars
• opacity
• hot and dense matter
• laboratory plasma
• line shapes
• Z-machine
• high energy density physics