A new theoretical model takes into account the rotation of the sun and the magnetic field

In the early 2000s, a new dataset revised the chemical abundances on the surface of the sun, contradicting the values ​​predicted by the standard models used by astrophysicists. Often disputed, these new abundances have been the subject of several new analyses. As they seemed to turn out to be correct, it was therefore up to the solar models to adapt, especially since they serve as a reference for the study of stars in general. A team of astronomers from the University of Geneva, Switzerland (UNIGE) in collaboration with the University of Liège, has developed a new theoretical model that solves part of the problem: consider the rotation of the sun, which has varied over time , and the magnetic fields it generates, they were able to explain the chemical structure of the sun. The results of this study are published in natural astronomy.

“The sun is the best characterizable star, so it is a fundamental test for our understanding of stellar physics. We have abundance measurements of its chemical elements, but also measurements of its internal structure. , as in the case of the Earth thanks to seismology”, explains Patrick Eggenberger, researcher at the UNIGE Department of Astronomy and first author of the study.

These observations should correspond to the results predicted by the theoretical models which aim to explain the evolution of the sun. How does the sun burn its hydrogen in the nucleus? How is energy produced there and then transported to the surface? How do chemical elements drift in the sun, influenced by both rotation and magnetic fields?

The standard solar model

“The standard solar model that we used until now considers our star in a very simplified way, on the one hand, with regard to the transport of chemical elements in the deepest layers; on the other hand, for the rotation and internal magnetic fields that have been totally neglected until now”, explains Gaël Buldgen, researcher at UNIGE’s Department of Astronomy and co-author of the study.

However, everything worked fine until the early 2000s, when an international scientific team drastically revised the solar abundances through improved analysis. The new abundances caused deep ripples in the waters of the solar model. Therefore, no model could reproduce the data obtained by helioseismology (the analysis of the oscillations of the sun), in particular the abundance of helium in the solar envelope.

A new model and the key role of rotation and magnetic fields

The new solar model developed by the UNIGE team includes not only the evolution of rotation, which was probably faster in the past, but also the magnetic instabilities it creates. “It is essential to consider simultaneously the effects of rotation and magnetic fields on the transport of chemical elements in our stellar models. It is important for the sun as well as for stellar physics in general and has a direct impact on the chemical evolution of the Universe, since the chemical elements crucial for life on Earth are cooked in the heart of the stars”, explains Patrick Eggenberger.

Not only does the new model correctly predict the concentration of helium in the outer layers of the sun, but it also reflects that of lithium, which has so far resisted modeling. “The abundance of helium is correctly reproduced by the new model because the internal rotation of the sun imposed by the magnetic fields generates a turbulent mixing which prevents this element from falling too quickly towards the center of the star; simultaneously, the abundance of lithium observed on the solar surface also reproduces because this same mixing transports it to the hot regions where it is destroyed”, explains Patrick Eggenberger

The problem is not fully resolved

However, the new model does not solve all the challenges raised by helioseismology: “Thanks to helioseismology, we know at less than 500 km in which region the convective movements of matter begin, at 199,500 km below the surface of the However, theoretical models of the sun predict a depth shift of 10,000 km”, explains Sébastien Salmon, researcher at UNIGE and co-author of the article. If the problem still exists with the new model, it opens a new door of understanding: “With the new model, we shed light on the physical processes that can help us resolve this critical difference.”

Updated Sun-type Stars

“We will have to review the masses, radii and ages obtained for the solar-type stars that we have studied so far”, explains Gaël Buldgen, detailing the next steps. Indeed, in most cases, solar physics is transposed to case studies close to the sun. Therefore, if the sun analysis models are changed, this update must also be done for other stars similar to ours.

Patrick Eggenberger specifies: “This is particularly important if we want to better characterize the host stars of planets, for example within the framework of the PLATO mission.” This observatory of 24 telescopes should fly to Lagrange 2 point (1.5 million kilometers from Earth, facing the Sun) in 2026 to discover and characterize small planets and refine the characteristics of their host star.