New Calculations Solve Mysteries of Solar Research

New Calculations Solve Mysteries of Solar Research
Editorial Board
/ Press release from the Max Planck Institute for Astronomy
May 23 2022

Evaluation of solar oscillations and the theory of stellar evolution, based on the chemical composition of the Sun, have so far provided contradictory data about the structure of the Sun. New calculations in solar atmospheric physics have now been able to resolve this discrepancy and provide corrected abundance values ​​for many chemical elements.

The Sun’s spectrum was recorded using the NARVAL High Resolution Spectrophotometer at Télescope Bernard Lyot of the Observatoire Midi-Pyrénées.

picture: M Bergmann/MBIA/Narval @TBL

What do you do when a proven method for determining the chemical composition of the Sun seems to conflict with an innovative and accurate technique for mapping the Sun’s structure? This was the situation astronomers faced in recent years when exploring the Sun – until new calculations published by Ekaterina Mag, Maria Bergman and their colleagues have now resolved the apparent contradiction.

The confirmed method in question is spectroscopy. In order to determine the chemical makeup of our sun or other stars, astronomers routinely use spectra: the decay of rainbow-style light into different wavelengths. Stellar spectra have prominent, sharp dark lines, first discovered by William Wollaston in 1802, rediscovered by Joseph von Fraunhofer in 1814, and recognized by Gustav Kirchhoff and Robert Bunsen in the 1860s as evidence of the presence of certain chemical elements.

The pioneering work of Indian astrophysicist Mijnad Saha in 1920 showed the quantitative relationship between the strength of “absorption lines”, star temperature and chemical composition. This provided the basis for our physical models of stars. Cecilia Payne-Gaposchkin’s discovery that stars like our sun consist primarily of hydrogen and helium, with only trace amounts of heavier chemical elements, builds on this work.

Fundamental calculations, concerning spectral properties on the one hand and chemical composition and stellar plasma physics on the other hand, have been of crucial importance to astrophysics since the time of Saha. They formed the basis of centuries of progress in understanding the chemical evolution of the universe, as well as reconstructing the physical structure and temporal evolution of stars and exoplanets. So it was a big shock when new observational data became available, providing insights into the inner workings of our Sun that didn’t quite match up with what we’ve reconstructed from the spectra.

The Modern Standard Model of Solar Evolution has been calibrated against a famous series (in heliophysics) of measurements of the chemical composition of the solar atmosphere, published in 2009. The new data are known as helios data, and they are measurements that capture very precisely the small oscillations of the Sun as a whole – the way it expands The surface of the Sun is contracting rhythmically in distinct patterns, on time scales ranging from seconds to hours. Just as seismic waves provide geologists with important information about the Earth’s interior, or the sound of a bell provides information about its shape and physical properties, heliosmology provides information about the Sun’s interior.

High-resolution measurements of the solar zyme have allowed conclusions to be drawn about the internal structure of the Sun that contradict standard models of our star’s structure based on solar chemistry. Specifically, according to helioscience, the so-called convection region inside our sun, where matter rises and falls like water in a pot, was much larger than the Standard Model predicted. The speed of sound waves in the lower regions of the convection zone also differed from what the Standard Model predicted, as is the total amount of helium in the Sun.

To make matters worse, certain measurements of solar neutrinos — ephemeral and elusive elementary particles that reach us directly from the Sun’s core regions — don’t quite fit the Standard Model. In astronomy one soon spoke of a “solar abundance crisis”, similar to the crisis of solar chemistry. Suggested solutions ranged from the unusual to the bizarre: Did the sun have accumulated mineral-poor gas during its planet formation stage? Is energy transfer in the interior of the sun by uninteracted dark matter particles actually?

The recently published study by a team led by Ekaterina Mag and Maria Bergman offers a solution that does not require any exotic physics. Instead, it provides a basic review of the models, on the basis of which the chemical composition of the solar spectrum is inferred. Early studies of this kind relied on the assumption of what is known as local thermal equilibrium: they assumed that the energy in every part of a star’s atmosphere had time to dissipate and reach some kind of equilibrium at each stage of its evolution. This means that a temperature can be assigned to each of these zones. This leads to a significant simplification of the calculations.

However, as early as the 1950s, some researchers realized that this image was too simplistic. Since then, the so-called non-LTE calculations have been performed in more and more studies, where the local thermal equilibrium (LTe) assumption has been omitted. Calculations other than LTE provide a detailed description of the energy exchange within the system – atoms are excited or collide with photons (particles of light), and photons are emitted, absorbed, or scattered. In stellar atmospheres where the density is too low for the system to reach thermal equilibrium, this level of detail pays off. There, calculations other than LTE give results that differ significantly from calculations that assume local thermal equilibrium.

The Maria Bergmann Group at the Max Planck Institute for Astronomy is a world leader in the application of non-LTE computations to the stellar atmosphere. As part of her PhD work on this group, Ekaterina Mag has set out to more precisely calculate the interaction of radiation with matter in the solar photosphere – the photosphere is the outermost layer of the solar atmosphere from which most externally radiant sunlight originates and where absorption lines are also sealed. in the solar spectrum.

In the study in question, the scientists looked at all of the chemical elements relevant to current models of star evolution. To make sure they had consistent results, the researchers applied several independent methods to describe the interactions between atoms and the sun’s radiation field. To describe the convective zones of our Sun, they used current simulations that take into account both the motion of the plasma and the physics of radiation. For comparison with the spectroscopic data, they chose the dataset of the highest quality available: the solar spectrum published by the Institute for Astrophysics and Geophysics at the University of Göttingen. “We have worked extensively to analyze the statistical and methodological effects that limit the accuracy of our results,” Mag says.

For some elements, the new calculations revealed a significantly different relationship between element abundance and the strength of the corresponding spectral lines than in previous work. Accordingly, significantly different chemical abundances appear when analyzing the observed solar spectrum compared to previous analyses. “We found that the proportion of elements heavier than helium in the Sun is 26% higher than what previous studies suggested,” Mag says.

These heavier elements are called “metals” in astronomy. Altogether, minerals make up a few thousandths of a percent of all atomic nuclei in the Sun. The best estimate for this value is now 26% higher than in previous studies. “The oxygen abundance value was approximately 15% higher than in previous studies,” Mag notes. The new values ​​align well with the chemistry of primitive meteorites (“CI chondrites”), which are thought to match that of the early Solar System.

By using the new values ​​as inputs to models of the structure and evolution of the Sun, the puzzling discrepancy between the results of those models and the seismic measurements of the Sun disappears. A comprehensive analysis of spectral line formation by Magg, Bergemann and their team, which is based on more complete models of fundamental physics than previous work, has shown how the ‘crisis’ can be overcome.

“New solar models, based on the new chemical composition values ​​we’ve identified, are becoming more realistic than ever: they produce a model of the Sun that matches all the information we have about the structure of the Sun today.—sound waves, neutrinos, luminosity, and radius of the Sun—without having to use Strange physics inside the sun,” confirms Bergman.

An additional advantage is that the new models can easily be applied to stars other than the Sun. While large-scale surveys such as the SDSS-V and 4MOST are yielding high-quality spectra of a growing number of stars, this kind of advance is already of great value – and provides a future analysis of stellar chemistry with its broader implications for reconstructing the chemical evolution of our universe on a more hardness than ever.

The team reports its findings in a specialized article published in the journal Astronomy and astrophysics He appeared.


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