Surprisingly different: Our sun contains more oxygen, silicon and neon than previously thought – the proportion of all heavier elements is even 26 percent higher than current models, as astronomers have discovered. This not only sheds new light on the structure and composition of our star, the new values also clarify a contradiction between spectral data and helioseismological measurements that has been puzzling for decades.
Astronomers use spectral analysis to find out which elements a star like our sun contains: Dark lines in the rainbow of the light spectrum reveal where atoms absorb parts of the radiation and which ones they are. In 1920, astrophysicists also discovered that the strength of these spectral lines also allowed conclusions to be drawn about the temperatures at their source. Since then, the solar spectrum has formed an important basis for models of the structure and development of the sun and other stars.
It was all the more shocking when astronomers realized a few years ago that these data and models did not match the results of helioseismology. With this relatively new measurement method, researchers use minute oscillations of the sun and its surface to draw conclusions about the processes and composition inside. However, the helioseismological observations deviate from those of the conventional models based on spectral data in several crucial points.
One of the discrepancies is that the layer in the sun shaped by convection currents must be much larger than the models predict. The speed of sound waves in the lower part of the convection zone, the total amount of helium and the release of solar neutrinos also show clear discrepancies with the spectral models. For years, astronomers have puzzled and debated this fundamental discrepancy, dubbed the “Solar Abundance Crisis.”
How can this contradiction between two fundamental and established methods of solar research be explained? Some researchers put forward rather exotic hypotheses, according to which our star is said to have swallowed metal-poor gas in its early days or that dark matter is hidden in the sun’s interior.
New evaluation of the solar spectrum
However, astronomers led by Ekaterina Magg from the Max Planck Institute for Astronomy in Heidelberg have chosen a different approach. So far, models based on spectral data have mostly assumed a local thermal equilibrium (LTE) in the interior of the sun. After this, the energy in each zone of the stellar atmosphere reaches an equilibrium that determines the local temperature. However, recent data suggest that this thermal equilibrium is not reached in many stellar atmospheres – the model is therefore oversimplified.
Magg and her team therefore used data from high-resolution solar spectra to more precisely calculate the interaction of radiation and matter in the solar photosphere using so-called non-LTE calculations. From these calculations, they re-determined the relationship between the strength of the spectral lines and the abundance of the corresponding element – and thus obtained new data on the chemical composition of the sun.
Sun is richer in metal than expected
The surprising result: The new results deviate significantly from the current models for several important elements. The sun apparently contains more oxygen, silicon and neon than previously thought. “The value for oxygen abundance was almost 15 percent higher than in previous studies,” reports Magg. Overall, the proportion of elements heavier than helium in the sun is even 26 percent higher than thought – the sun is therefore richer in metal than earlier assumptions.
More importantly, these new values resolve the “Solar Abundance Crisis”: Inserting the spectral element values into the models of the structure and evolution of the Sun, the puzzling discrepancy with the helioseismic measurements disappears. “This is the first time that standard solar models based on spectroscopic results can reproduce the inner structure of the Sun, which was determined by helioseismological techniques,” states the research team.
Way to better solar models
The study thus paves the way to new models of the Sun and stellar structure that are compatible with spectral and helioseismological observations. “The new solar models, based on the new chemical composition values we have determined, are more realistic than ever before: they result in a model of the sun that is compatible with all the information we have about the structure of the sun today – sound waves, neutrinos , luminosity and solar radius – without having to use exotic physics inside the sun,” says Magg’s colleague Maria Bergemann.
Another advantage is that the new models can also be better applied to stars other than the sun. This puts future analyzes of stellar chemistry, and hence reconstructions of the chemical evolution of our cosmos, on a more solid footing than ever before. (Astronomy & Astrophysics, 2022; doi: 10.1051/0004-6361/202142971)
Source: Max Planck Institute for Astronomy
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