New Calculations Composition Solar Spectrum Resolve
What do you do when a tried-and-true way to figure out the chemical makeup of the sun seems to be at odds with a new, precise way to map the sun’s inner structure? That’s what astronomers who study the sun had to deal with until Ekaterina Magg, Maria Bergemann, and others published new calculations that explain the apparent contradiction.
The solar abundance crisis has been going on for more than a decade. It is caused by a disagreement between the structure of the sun’s interior as shown by solar oscillations (helioseismology) and the structure shown by the basic theory of stellar evolution, which is based on measurements of the sun’s chemical makeup today. The conflict is solved by the new calculations of the physics of the sun’s atmosphere, which give new numbers for the amounts of different chemical elements. In particular, the sun has more oxygen, silicon, and neon than was thought before. The methods also promise much more accurate estimates of the chemicals that make up stars in general.
Spectra are used in astrochemistry.
Spectral analysis is the tried-and-true method in question. Astronomers often use spectra, which is the way light is broken up into its different wavelengths, to figure out the chemical makeup of our sun or any other star out there. William Wollaston first noticed dark, sharp lines in the stellar spectrum in 1802. Joseph von Fraunhofer rediscovered them in 1814, and in the 1860s, Gustav Kirchhoff and Robert Bunsen figured out that these lines were signs that certain chemical elements were present.
Meghnad Saha, an Indian astronomer, did groundbreaking work in 1920 when he linked the strength of these “absorption lines” to the temperature and chemical makeup of stars. This laid the groundwork for our physical models of stars. Based on that work, Cecilia Payne-Gaposchkin found that stars like our sun are mostly made of hydrogen and helium, with only small amounts of heavier chemical elements.
There are changes in the sun that tell a different story.
Since then, astrophysics has relied heavily on the calculations that link spectral features to the chemical makeup and physics of the stellar plasma. They have helped us learn more about the chemical evolution of the universe as well as the physical structure and evolution of stars and exoplanets over the past 100 years. That’s why it was a bit of a surprise when, as new observational data became available and showed how our sun works on the inside, the puzzle pieces didn’t seem to fit together.
The modern standard model of how the sun changes are based on a set of measurements of the chemical composition of the sun’s atmosphere that was published in 2009. But in a number of important ways, a reconstruction of our favorite star’s inner structure based on the standard model goes against another set of measurements: helioseismic data. These are measurements that track very precisely the minute oscillations of the sun as a whole—the way the sun expands and contracts in rhythmic patterns on time scales between seconds and hours.
Helioseismology tells us about the inside of the sun in the same way that seismic waves tell geologists important things about the inside of the earth or that the sound of a bell tells us about its shape and material properties.
The crisis of the sun’s plenty
Helioseismic measurements that were very accurate showed that the standard models of the sun’s inside structure were wrong. Helioseismology found that the so-called convective region of our sun, where matter rises and falls like water in a boiling pot, was much bigger than the standard model predicted. Both the speed of sound waves and the total amount of helium in the sun were different from what the standard model said they should be. On top of that, some measurements of solar neutrinos, which are elementary particles that move quickly and are hard to detect, come directly from the sun’s core regions and are hard to measure.
Astronomers had what they soon came to call a “solar abundance crisis,” and as they looked for a way out, they came up with ideas that ranged from strange to outlandish. During the time it was making planets, did the sun maybe add some gas that didn’t have much metal? Are dark matter particles, which are known for not interacting, moving energy?
Calculations outside of local thermal equilibrium
The new study by Ekaterina Magg, Maria Bergemann, and others has solved this problem by looking again at the models that are used to estimate the chemical makeup of the sun’s spectrum. When people first tried to figure out how stars made their spectra, they used something called “local thermal equilibrium.” They thought that energy in each part of a star’s atmosphere would have time to spread out and reach a sort of balance. This would make it possible to give each of these places a temperature, which would make the calculations much easier.
But astronomers knew as early as the 1950s that this picture was too simple. Since then, more and more studies have used what are called “non-LTE” calculations instead of assuming that everything is in balance locally. In the non-LTE calculations, there is a detailed explanation of how energy is transferred within the system, such as when photons excite atoms or when photons hit each other and emit, absorb, or scatter. In stellar atmospheres, where densities are much too low for the system to reach thermal equilibrium, this kind of attention to detail pays off. There, calculations that don’t use local equilibrium give results that are very different from those that do.
Using non-LTE on the photosphere of the sun
The group led by Maria Bergemann at the Max Planck Institute for Astronomy is one of the best in the world at using non-LTE calculations to study stellar atmospheres. Ekaterina Magg wanted to figure out more about how radiation and matter interact in the solar photosphere as part of her Ph.D. work in that group. Most of the sun’s light comes from the photosphere, which is the outermost layer. It is also where the absorption lines on the solar spectrum are made.
In this study, they kept track of all the chemical elements that are important to the current models of how stars have changed over time. They also used more than one way to describe how the sun’s atoms interact with its radiation field to make sure their results were consistent. They used simulations that already existed and took into account both the movement of the plasma and the physics of radiation to describe the convective parts of our sun (“STAGGER” and “CO5BOLD”). For the comparison with spectral measurements, they chose the data set with the best quality: the solar spectrum published by the Institute for Astro- and Geophysics, University of Gottingen. Magg says, “We also paid a lot of attention to analyzing statistical and systematic effects that could make our results less accurate.”
A sun with more oxygen and more heavy stuff
The new calculations showed that what previous authors said about the relationship between the amount of these important chemical elements and the strength of their spectral lines was very wrong. Because of this, the chemical abundances that follow from the observed solar spectrum aren’t exactly the same as what was said in earlier studies.
Magg says, “Our analysis showed that the sun has 26% more elements heavier than helium than what previous studies had thought.” In astronomy, “metals” are all things that are heavier than helium. Only about a thousandth of one percent of the atomic nuclei in the sun are metals. This very small number has changed by 26% from what it was before. Magg says, “The value for the amount of oxygen was about 15% higher than what was found in earlier studies.” The new values, on the other hand, are in good agreement with the chemical make-up of primitive meteorites (“CI chondrites”), which are thought to reflect the chemical make-up of the solar system when it was very young.
When these new values are put into the current models of how the sun is built and how it changes, the strange difference between the models’ results and helioseismic measurements goes away. The solar abundance crisis is solved by Magg, Bergemann, and their colleagues’ detailed analysis of how spectral lines are made, which is based on much more complete models of the physics at work.
Maria Bergemann says, “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 current structure—sound waves, neutrinos, luminosity, and the sun’s radius—without the need for non-standard, exotic physics in the solar interior.”
Also, the new models are easy to use for stars other than the sun, which is a plus. At a time when large-scale surveys like SDSS-V and 4MOST are giving high-quality spectra for an increasing number of stars, this kind of progress is very important. It puts future analyses of stellar chemistry, which are important for figuring out how the chemistry of the universe has changed over time, on a stronger foundation than ever before.
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