In the early 2000s, a new set of data revised the chemical abundances at the
surface of the sun, contradicting the values predicted by the standard
models used by astrophysicists. Often challenged, these new abundances made
it through several new analyses. As they seemed to prove correct, it was
thus 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
Université de Liège, has developed a new theoretical model that solves part
of the problem: considering the sun's rotation, that varied through time,
and the magnetic fields it generates, they have been able to explain the
chemical structure of the sun. The results of this study are published in
Nature Astronomy.
"The sun is the star that we can best characterize, so it constitutes a
fundamental test for our understanding of stellar physics. We have abundance
measurements of its chemical elements, but also measurements of its internal
structure, like in the case of Earth thanks to seismology," explains Patrick
Eggenberger, a researcher at the Department of astronomy of the UNIGE and
first author of the study.
These observations should fall in line with the results predicted by the
theoretical models which aim at explaining the sun's evolution. How does the
sun burn its hydrogen in the core? How is energy produced there and then
transported towards the surface? How do chemical elements drift within the
sun, influenced both by rotation and magnetic fields?
The standard solar model
"The standard solar model we used until now considers our star in a very
simplified manner, on the one hand, with regard to the transport of the
chemical elements in the deepest layers; on the other hand, for the rotation
and the internal magnetic fields that were entirely neglected until now,"
explains Gaël Buldgen, a researcher at the Department of astronomy of the
UNIGE and co-author of the study.
However, everything worked fine until the early 2000s, when an international
scientific team drastically revised the solar abundances thanks to an
improved analysis. The new abundances caused deep ripples in the waters of
the solar modeling. From then on, no model was able to reproduce the data
obtained by helioseismology (the analysis of the sun's oscillations), 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. "We must absolutely 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
for stellar physics in general and has a direct impact on the chemical
evolution of the Universe, given that the chemical elements that are crucial
for life on Earth are cooked in the core of the stars," says Patrick
Eggenberger.
Not only does the new model rightly predict the concentration of helium in
the outer layers of the sun, but it also reflects that of lithium which
resisted modeling until now. "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 is
also reproduced 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 doesn't solve every challenge raised by
helioseismology: "Thanks to helioseismology, we know within 500 km in which
region the convective movements of matter begin, 199,500 km below the
surface of the sun. However, the theoretical models of the sun predict a
depth offset of 10,000 km," says Sébastien Salmon, researcher at the UNIGE
and co-author of the paper. If the problem still exists with the new model,
it opens a new door of understanding: "Thanks to the new model, we shed
light on the physical processes that can help us resolve this critical
difference."
Update of solar-like stars
"We are going to have to revise the masses, radii and ages obtained for the
solar-type stars that we have studied so far," says Gaël Buldgen, detailing
the next steps. Indeed, in most cases, solar physics is transposed to case
studies close to the sun. Therefore, if the models for analyzing the sun are
modified, this update must also be performed for other stars similar to
ours.
Patrick Eggenberger says: "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 the Lagrange point 2 (1.5 million kilometers from Earth, opposite the
sun) in 2026 to discover and characterize small planets and refine the
characteristics of their host star.
Reference:
P. Eggenberger et al, The internal rotation of the Sun and its link to the
solar Li and He surface abundances, Nature Astronomy (2022).
DOI: 10.1038/s41550-022-01677-0
Tags:
Space & Astrophysics