When a tried-and-true method for determining the sun's chemical composition appears to be at odds with an innovative, precise technique for mapping the sun's inner structure, what do you do? New calculations published by Maria Bergemann and colleagues resolve the apparent contradiction, as they were the first to publish them.
There is a conflict between the internal structure of the sun and the structure derived from the fundamental theory of stellar evolution, which is the reason for the decade-long solar abundance crisis. The new calculations of the physics of the sun's atmosphere yield updated results for abundances of different chemical elements which resolve the conflict. The sun has more oxygen, Silicon and Neon than previously thought. Estimates of the chemical compositions of stars in general will be vastly improved by the methods employed.
Astrochemistry using a spectrum.
The method in question is a tried and true one. Calculating the chemical composition of our sun or any other star is done by using the rainbow-like decomposition of light into its different wavelength. The first noticed by William Wollaston in 1802 was a sharp dark line, and it was rediscovered by Joseph von Fraunhofer in 1814.
The basis for our physical models of stars was provided by the work of the Indian astrophysicist Meghnad Saha in 1920. The realization that stars like our sun are mostly hydrogen and helium with no heavier chemical elements is based on that work.
There are solar oscillations that tell a different story.
The calculations relating to the chemical composition and physics of the stellar plasma have been important to astrophysics ever since. They have been the foundation of a century-long progress in our understanding of the chemical evolution of the universe as well as of the physical structure and evolution of stars and exoplanets. It came as a bit of a shock when it was discovered that the different pieces of the puzzle did not fit together.
A famous set of measurements of the solar atmosphere was published in 2009. A reconstruction of the inner structure of our favorite star, based on that standard model, is contrary to other measurement of the sun's activity.
Similar to the sound of a bell, helioseismology provides important information about the interior of the sun.
There is a solar abundances crisis.
The sun's interior structure was found to be at odds with the solar standard models. 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 amount of helium in the sun and the speed of sound waves near the bottom of that region deviated from the model predictions. To top it all off, certain measurements of solar neutrinos were slightly off compared to experimental data.
Astronomers had a solar abundances crisis, and in search of a way out, some proposals ranged from the unusual to the downright exotic. Did the sun make metal-poor gas during its planet-forming phase? Is the energy being transported by dark matter particles?
Local thermal equilibrium is beyond the calculations.
The models on which the sun's chemical composition are based were reexamined by the authors of the new study. Local thermal equilibrium was used in early studies of how the stars are produced. They assumed that the energy in each region of a star has time to spread and reach a kind of equilibrium. It would be possible to assign a temperature to each region, which would lead to a lot of simplification in the calculations.
The picture was oversimplified as early as the 1950s. The assumption of local equilibrium has been dropped by more and more studies. The Non-LTE calculations include a detailed description of how energy is exchanged within the system. In stellar atmospheres, where densities are too low to allow the system to reach thermal equilibrium, that kind of attention to detail pays off. The results of Non-LTE calculations are vastly different from those of their local-equilibrium counterparts.
The solar photoosphere is being applied to.
Maria Bergemann is a leader in applying Non-LTE calculations to stellar atmospheres. As part of her PhD work, Magg set out to calculate the interaction of radiation matter in the solar photoosphere. The absorption lines on the solar spectrum are imprinted on the photoosphere, where most of the sun&s light comes from.
They 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, after they tracked all chemical elements that are relevant to the current models of how stars evolved over time. The convective regions of our sun were described using existing simulations that took into account both the motion of the plasma and the physics of radiation. The Institute for Astro- and Geophysics, University of Gttingen, published the highest quality solar spectrum data for comparison.
The sun has more oxygen and heavier elements.
The new calculations showed that the relationship between the abundances of these chemical elements and the strength of the corresponding lines was different from what previous authors had claimed. The chemical abundances that follow from the observed solar spectrum are different from what was previously stated.
According to our analysis, the sun contains 26% more elements heavier than helium than had been thought. 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 changed by 26% of its previous value. The value for the oxygen abundance was almost 15% higher than in previous studies.
The crisis has been resolved.
The puzzling discrepancy between the results of the models and the helioseismic measurements disappears when those new values are used. The in-depth analysis by Magg, Bergemann and their colleagues of how spectral lines are produced, with its reliance on more complete models of the underlying physics, resolves the solar abundance crisis.
The new solar models based on our new chemical composition are more realistic than ever before, they produce a model of the sun that is consistent with all the information we have about the sun's present-day structure.
The new models are easy to apply to stars other than the sun. Future analyses of stellar chemistry, with their broader implications for reconstructions of the chemical evolution, are a result of this kind of progress.
Observational constraints on the origin of elements are studied. The standard composition of the sun is published in the journal Astronomy and Astrophysics.
More information: Ekaterina Magg et al, Observational constraints on the origin of the elements, Astronomy & Astrophysics (2022). DOI: 10.1051/0004-6361/202142971 Journal information: Astronomy & Astrophysics Citation: New calculations of solar spectrum resolve decade-long controversy about the sun's chemical composition (2022, May 23) retrieved 23 May 2022 from https://phys.org/news/2022-05-solar-spectrum-decade-long-controversy-sun.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
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