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May 24, 2022 07:00 am

New Calculations of Solar Spectrum Resolve Decade-Long Controversy About the Sun's Chemical Composition

Researchers from the University of Gottingen have published new calculations of the physics of the sun's atmosphere that resolve the apparent contradiction between the modern standard model of solar evolution and the "tried-and-true" method called spectral analysis. "The new calculations of the physics of the sun's atmosphere yield updated results for abundances of different chemical elements, which resolve the conflict," reports Phys.Org. "Notably, the sun contains more oxygen, silicon and neon than previously thought. The methods employed also promise considerably more accurate estimates of the chemical compositions of stars in general." From the report: Highly accurate helioseismic measurements gave results about the sun's interior structure that were at odds with the solar standard models. According to helioseismology, the so-called convective region within our sun where matter rises and sinks down again, like water in a boiling pot, was considerably larger than the standard model predicted. The speed of sound waves near the bottom of that region also deviated from the standard model's predictions, as did the overall amount of helium in the sun. To top it off, certain measurements of solar neutrinos -- fleeting elementary particles, hard to detect, reaching us directly from the sun's core regions -- were slightly off compared to experimental data, as well. Astronomers had what they soon came to call a "solar abundances crisis," and in search of a way out, some proposals ranged from the unusual to the downright exotic. Did the sun maybe accrete some metal-poor gas during its planet-forming phase? Is energy being transported by the notoriously non-interacting dark matter particles? The newly published study by Ekaterina Magg, Maria Bergemann and colleagues has managed to resolve that crisis, by revisiting the models on which the spectral estimates of the sun's chemical composition are based. [...] In this study they tracked all chemical elements that are relevant to the current models of how stars evolved over time, and applied multiple independent methods to describe the interactions between the sun's atoms and its radiation field in order to make sure their results were consistent. For describing the convective regions of our sun, they used existing simulations that take into account both the motion of the plasma and the physics of radiation ("STAGGER" and "CO5BOLD"). For the comparison with spectral measurements, they chose the data set with the highest available quality: the solar spectrum published by the Institute for Astro- and Geophysics, University of GÃttingen. "We also extensively focused on the analysis of statistical and systematic effects that could limit the accuracy of out results," notes Magg. The new calculations showed that the relationship between the abundances of these crucial chemical elements and the strength of the corresponding spectral lines was significantly different from what previous authors had claimed. Consequently, the chemical abundances that follow from the observed solar spectrum are somewhat different than stated in previous analysis. "We found, that according to our analysis the sun contains 26% more elements heavier than helium than previous studies had deduced," explains Magg. In astronomy, such elements heavier than helium are called "metals." Only on the order of a thousandth of a percent of all atomic nuclei in the sun are metals; it is this very small number that has now changed by 26% of its previous value. Magg adds: "The value for the oxygen abundance was almost 15% higher than in previous studies." The new values are, however, in good agreement with the chemical composition of primitive meteorites ("CI chondrites") that are thought to represent the chemical make-up of the very early solar system. When those new values are used as the input for current models of solar structure and evolution, the puzzling discrepancy between the results of those models and helioseismic measurements disappears.

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