Gaël Buldgen, Sébastien Salmon and Arlette Noels from the Theoretical Stellar Astrophysics and Asteroseismology Laboratory (ASTA) of the STAR Institute of the University of Liege have developed new seismic sounding techniques for the study of our Sun. Built using the new chemical abundances of the Sun - determined by 3D simulations of its outer layers - the new solar model seriously questions our knowledge of the Sun, but will also impact several studies of stellar systems for which the previous solar model, more than 20 years old, served as a reference. The results of these researches, which have been published almost simultaneously in the journals Astronomy & Astrophysics (1) and Monthly Notices of the Royal Astronomical Society (2,3), are currently the object of many discussions in the international scientific community.
T
he Sun, our star, is the engine of our planetary system and a key actor for the processes that led to the appearance of life on Earth. The study of its composition and behaviour is therefore essential in order to better understand the evolution of our planetary system. The last solar models, established in the 1990s, are however questioned today by researchers of the University of Liège. These scientists criticise them for not including the latest solar chemical abundance and therefore not providing an accurate depiction of the current state of the Sun and its internal structure. "It is important to notice that the studies of many other stars are based on models developed for the study of the Sun, also called standard models whose "recipe" was defined by John Bahcall in the 1980s," says Gaël Buldgen, a researcher of the ASTA laboratory, currently a post-doctoral fellow in the HiROS laboratory at the University of Birmingham. "The Sun is a privileged laboratory for the detailed study of the evolution and structure of stars in general. It is therefore important to have accurate models, in line with the most recent observations ". It was during his PhD thesis in Liège that this young researcher imagined and developed new seismic sounding techniques that clearly showed that models computed using old solar abundances (the content of different chemical elements) were no longer in agreement with helioseismic data. This result, defended by researchers at the University of Liège, caused some jolts in the international scientific community.
"Helioseismology was born in the late 1960s, when scientists first observed oscillations at the Sun surface" explains Sébastien Salmon, a researcher at the ASTA (UR STAR) laboratory. "Similarly to seismology of the Earth, which allows us to probe the structure of our planet by studying the waves produced during earthquakes, this discipline of astrophysics provides the keys to unravel the internal structure of our star by analysing the oscillations detected on the Sun's surface." Thanks to ground-based observatory networks, such as the GONG (Global Oscillations Network Group) and BiSON (Birmingham Solar Oscillations Network), or space-based observatories such as SoHO (Solar and Heliospheric Observatory), a satellite placed in orbit around the Sun, scientists have since been able to detect more than 100.000 frequencies of oscillations which, like musical notes, allow us to reconstruct the structure and properties of the instrument that produces them.
Upper panel, the Soho satellite has proven to be one of the most effective instruments to detect the oscillations of the Sun's surface, giving us access to its internal structure using Helioseismology (artist's view)[Credit: SOHO (ESA & NASA)]. Lower panel: Latitudinal section of the Sun. The propagation of oscillation modes within the Sun is represented by the black curves. The modes used in the work of the Liège team are the pressure modes, known as p-modes, which probe mostly the Sun's envelope. Gravity modes, known as g-modes, probe the star's core, and were detected only very recently (July 2017) by the team of researcher Eric Fossat. [Credit: ESA; Sun's chromosphere based on SOHO image; credit: SOHO (ESA & NASA)
In the 1990s, helioseismology was thus able to determine very precisely the boundary between the convective zone (area below the Sun's surface where energy is transported by movements of matter) and the radiative zone (deeper zone where energy is transported by radiation) of the Sun.
The progress of this discipline has allowed scientists to probe the physical conditions reigning more than 660,000 km below the visible surface of our star, a distance representing 95% of its radius! In such analyses, the question of chemical abundances is paramount. Indeed, the structure and evolution of a star strongly depends on the relative proportion of each of its various chemical elements. Hydrogen is the main component (74% of the mass of our star), followed by helium (approximately 24%) and other so-called "heavy" elements (carbon, neon, oxygen, iron, magnesium, silicon, etc.). Solar abundances generally serve as standards for the study of stellar spectra. "We calculate them from the analysis of spectral lines produced in the photosphere, i. e. in the surface layers of the Sun," explains Arlette Noels, scientific collaborator at the ASTA laboratory. "Spectroscopy is one of the main tools used by astrophysicists to study our Universe, as a spectral analysis delivers a significant amount of information on the emitting source. However, in order to obtain reliable results, Arlette Noels continues, it is necessary to have detailed models that provide the physical conditions and processes at work within these superficial layers. Crucial progress has been made in this area over the last few years, which has prompted a reanalysis of the solar abundances". In order to construct a theoretical model of a star, we need to know its chemical composition, i. e. the various elements that make up its structure, as well as the physical conditions and processes within it. "The Sun is the most studied of all stars in the Universe. Thus, the tools and models developed for the Sun are now being applied to the study of other stars," says Sébastien Salmon.
View of the Sun in multiple bandwidths (a bandwidth corresponds to a certain wavelength interval, i. e. a certain "colour"). It is by observing the Sun in different combinations of bandwidths that we can detect and analyse the oscillations of its surface. Similarly, spectroscopy (very narrow bandwidths, centred on very precise wavelengths) allows scientists to analyse the spectral lines of the Sun's photosphere, and to deduce the abundance of the various chemical elements on its surface. [Credit: NASA/SDO/Goddard Space Flight Center]
In 2005, thanks to the progress made in the analysis of these spectroscopic observations, researchers, including one from Liège, Nicolas Grevesse, who was directly involved in these new analyses (as well as in the old determinations that led to the success of solar models and which were carried out in collaboration with Arlette Noels), deduced a drastic reduction in the carbon, oxygen and nitrogen abundances of around 30%. However, these new observations seem to provide solar models in stark disagreement with helioseismology! "Since then, scientists have been trying unsuccessfully to reconcile these theoretical models with helioseismic data," says Sébastien Salmon. "The scientific world thus experiences a crisis, "a trouble in paradise", which leads many scientists to simply ignore the new abundances rather than question the theoretical model".
At the University of Liège, researchers would have none of it and decided to re-examine the problem. Gaël Bulgen, back then a PhD student in the ASTA group, worked with his colleagues on the development of new seismic probing techniques. « We decided to tackle this problem by adopting a completely different approach, by "listening" to the solar oscillations with new helioseismic tools, » explains the young researcher. Using these new techniques, Gaël Buldgen determined by seismic analysis the content of elements heavier than helium in the solar matter. And the resulting value is compatible with the "new" abundances! It is thus urgent to definitively adopt the new abundances and carry out more detailed studies, which are essential for a better understanding of the structure of our Sun, and thus define theoretical models much more coherent and realistic from a physical point of view.
In this work, our researchers also determined the internal profile of the solar entropy and gain a new vision of the layers separating the radiative region from the convective envelope. Their results now clearly show the weaknesses of the models constructed with the old abundances, which do not reproduce their new helioseismic constraints. However, the scientific community remains cautious to validate the implementation of a new solar model: it not only requires a strong revision of the "recipe" used to build it, but - given the use of this model for the characterization of other stars – its revision can also lead to important changes in the determination of mass, radius and age of other solar-like stars, for e.g. Alpha Centauri (one of the most promising stellar system to look for hints of life...). The precise characterisation of masses, radii and ages of stars have been a matter of priority of stellar physicists for a decade, thanks in particular to the CoRoT and Kepler satellites, which have made it possible to study thousands of stars with an asteroseismology. This race for precision is at the centre of the future PLATO mission of the European Space Agency (ESA), which main prerequisites are the determination of these stellar parameters with accuracies below 10%.
In this sense, the work of the Liège team is at the heart of the scientific community's preoccupations because it addresses the basis of stellar model reliability. Beyond the field of stellar astrophysics itself, stellar models are also used to characterise exoplanetary systems and study the chemical evolution of our Galaxy. Any change in their description has thus inevitable repercussions in other fields of astrophysics, as soon as they use information obtained by evolutionary stellar models.
Z machine laser at the Sandia National Labortatory (USA), one of the most powerful lasers in the world. To date, it is the only one that has been able to reproduce temperature and density conditions similar to the deep regions of the Sun. A plasma of about 1,000,000°C was produced there to study the atomic properties of solar plasma. [Credit: R. Montoya/Sandia National Laboratories]
This new work has also new perspectives on the study of transition regions within stars, where energy transport changes from one regime to another. Turbulent motions in these regions are poorly understood and modelled in theoretical models. The techniques proposed by the ASTA laboratory offer new and strong constraints on these layers. Furthermore, these processes are also linked to fundamental atomic physics phenomena and in this area, a new constraint is offered by experiments with the most powerful existing experimental lasers. Research in helioseismology may thus be somewhat linked to study of extreme physical conditions necessary for controlled nuclear fusion on Earth, one of the potential, and so crucial, sources of energy production for our societies in the future.
« We need to continue the work and highlight the sensitive points of other solar models, » concludes Sébastien Salmon. « It is essential to precisely reduce the remainingdiscrepancies in order to avoid new conceptual errors. » The researchers are continuing their effort and are already collaborating with numerous scientific groups. The University of Birmingham, the Geneva Observatory and the American laboratory of Los Alamos joined them to re-analyse in detail the structure of our Sun and determine its impact on other fields of astrophysics.
(1) Buldgen, Gaël ; Salmon, Sébastien ; Noels-Grötsch, Arlette et al., Seismic inversion of the solar entropy. A case for improving the standard solar model, in Astronomy and Astrophysics (2017), 607
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(2) Buldgen, Gaël ; Salmon, Sébastien ; Noels-Grötsch, Arlette et al., Determining the metallicity of the solar envelope using seismic inversion techniques, in Monthly Notices of the Royal Astronomical Society (2017), 472(1), 751-764
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(3) Buldgen, Gaël ; Salmon, Sébastien ; Godart, Mélanie et al., Inversions of the Ledoux discriminant: a closer look at the tachocline, in Monthly Notices of the Royal Astronomical Society: Letters (2017), 472(1), 70-74
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Laboratoire d’Astrophysique Stellaire Théorique et Astérosismologie(ASTA) - Unité de Recherche STAR
Gaël BULDGEN: gbuldgen@uliege.be
Sébastien SALMON : Sebastien.Salmon@uliege.be
Arlette NOËLS : Arlette.Noels@uliege.be
Gaël Buldgen holds a Bachelor's degree in Physical Sciences and a Master's degree in Space Science of the Department of Astrophysics, Geophysics and Oceanography from the University of Liège. He did his PhD in the Theoretical Stellar Astrophysics and Asterosismology Laboratory (ASTA), studying the development of inversion techniques in asteroseismology, under the supervision of Marc-Antoine Dupret (ULiège) and Daniel Reese (Observatoire de Paris-Meudon). Gaël Buldgen is currently a post-doctoral fellow in the HiROS group (High-Resolution Optical Spectroscopy) of the University of Birmingham where he is working on the application of the methods developed during his PhD to more evolved stars such as sub-giants and red giants, as well as on new generations of solar models.
After completing a bachelor's degree in physics at UCL, Sébastien Salmon came to ULiège to pursue a master's degree in space sciences, the only French-speaking Belgian university to offer this type of study. His master thesis was awarded the Wallonie Espace prize. He then carried out a PhD thesis on the study of massive stars and their oscillations. His research highlighted phenomena on the atomic scale within these stars. After his doctoral thesis, he left for a post-doc at the CEA, France's Atomic Energy Research Center, partly devoted to astrophysical experiments with the world's most energetic lasers. He is now involved in the scientific preparation of the exoplanetology satellite CHEOPS.
Arlette Noels began her career in the sixties in the group of Professor Paul Ledoux, an internationally renowned specialist in internal structure and star stability. At the time, stellar astrophysics was in its infancy. Attracted by the innovative concept of computing a detailed stellar model, she naturally dedicated her thesis and subsequent research to theoretical stellar astrophysics. Later on she became professor at the University of Liège and she supervised several doctoral theses and directed a group of about ten researchers. After having had the chance to experience the exciting era in which the theories of stellar evolution revealed their secrets, Professor Arlette Noels embarked on a new stellar epic focused on extremely challenging observations that can unveil the interiors of stars, thanks to asteroseismology.
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