New insights into solar opacity through helioseismology

New insights into solar opacity through helioseismology
by Erica Marchand
Paris, France (SPX) Jan 29, 2025

Helioseismology, the study of the Sun's acoustic oscillations, provides a powerful tool for probing the internal properties of our star with remarkable precision. By analyzing these sound waves, scientists can reconstruct critical parameters such as the Sun's density, temperature, and chemical composition. This transformative method enables researchers to explore the Sun's inner workings as an astrophysical laboratory, refining stellar models and advancing our understanding of stellar evolution in the universe.

A recent international study, led by Gael Buldgen from the ULiege Institute of Astrophysics and Geophysics, used helioseismic methods to independently measure how high-energy radiation is absorbed by solar plasma in the Sun's deep interior.

This groundbreaking research illuminates solar radiative opacity, a key factor governing the interaction of matter and radiation under extreme solar conditions. The study validates prior findings from renowned facilities such as Sandia National Laboratories and Livermore National Laboratory, while identifying unresolved challenges in atomic physics and disparities among predictive models from Los Alamos National Laboratory, Ohio State University, and CEA-Saclay.

ULiege's Precision Tools Advance Stellar Modeling

The ULiege team utilized advanced numerical techniques rooted in the university's expertise in helioseismology and stellar modeling. "By detecting the Sun's acoustic waves with extraordinary accuracy, we can deduce internal properties of our star, much like determining the characteristics of a musical instrument from the sounds it produces," said Gael Buldgen.

Helioseismic measurements achieve a level of precision that allows for estimating the mass of a cubic centimeter of solar matter with accuracy surpassing that of a high-precision kitchen scale, all without direct observation or physical interaction. Since its inception in the late 20th century, helioseismology has significantly influenced fundamental physics, contributing to critical discoveries such as neutrino oscillations, a phenomenon recognized by the 2015 Nobel Prize.

However, revisions to the solar chemical composition in 2009 - confirmed in 2021 with contributions from ULiege researchers - introduced challenges to solar models, which no longer aligned with helioseismic data.

To address these discrepancies, the University of Liege developed advanced modeling tools as part of doctoral research initiatives, later refined through collaborations with institutions in Birmingham and Geneva. This work revisited the Sun's internal thermodynamic conditions, reigniting interest in solar radiative opacity. Research led by James Bailey at Sandia National Laboratory in 2015 underscored the pivotal role of radiative opacity, though initial experimental results revealed significant gaps when compared to theoretical models.

Economic and Practical Benefits of Helioseismic Measurements

Helioseismic data provide a cost-effective alternative to laboratory experiments, such as those conducted with the Z Machine, which require substantial energy and financial resources. These measurements not only validate experimental findings but also guide future experiments by identifying temperature, density, and energy regimes that more accurately replicate solar conditions.

The implications of this research extend beyond the Sun, enhancing the theoretical models used to determine the age and mass of stars and exoplanets. "The Sun is our benchmark for stellar evolution, a key laboratory for verifying our understanding. These results gain even greater importance as we prepare to launch the PLATO satellite in 2026, aimed at characterizing solar-type stars to identify habitable planets," added Gael Buldgen.

"Moreover, these findings resonate with nuclear fusion research, offering insights into the Sun's conditions that can inform the development of fusion energy - a critical pursuit for clean energy solutions."

A Call for Improved Atomic Models

This study underscores the necessity of refining atomic models to resolve discrepancies between experimental data and theoretical calculations. Such advancements promise to redefine stellar evolution theories and deepen our comprehension of the physical processes shaping stars. The research highlights the University of Liege's leading role in astrophysical science, showcasing the transformative potential of helioseismology in unraveling cosmic mysteries.

Seismic inversion of the solar entropy. A case for improving the standard solar model, Astronomy and Astrophysics

https://doi.org/10.1051/0004-6361/201731354