Research
The “life” of Stars
In the team ASTA, our objects of study are the stars. Numerous and bright, they offer us the richest information about our universe. Stars evolve due to thermonuclear reactions in their core synthesizing vital elements of the universe. For example, our Sun was formed 4.6 milliards of years ago. In 7 milliards years, it will become a red giant and synthesize carbon and oxygen in its core. Finally, it will lose its envelope and become a slowly cooling white dwarf. Based on our knowledge of this evolution, the age of stars can be determined, opening the door to the knowledge of the evolution of our Galaxy and exoplanetary systems. The contribution of ASTA to the study of stellar evolution is at a theoretical level, we solve the physical laws to which they must obey, building models of stars describing their internal structure and their evolution, with the Liège Stellar Evolution Code (CLES).
Mysterious transport processes
Although the main tracks of stellar evolution are now identified, a bunch of clues points towards the presence of mysterious transport processes acting in their hidden interior by modifying their chemical composition and internal rotation. What are these processes and the laws to which they obey: meridional currents and turbulence generated by rotation ? transport by waves ? Larger extension of convective zones ? How is the magnetic field interacting with these processes ? Answering these questions is essential. Indeed, these processes modify the quantity of material transported to the stellar core where they are burned by nuclear reactions, extending the lifetime of stars. The transport of energy throughout stars is also a source of questioning: the accurate modeling of turbulent motions in convective zones remains very difficult and the transport of energy by radiation is clouded by the uncertain opacity of stellar matter.
Listening to the stars: asteroseismology
Stars are completely opaque, we only see their surface. How to unveil the mysterious transport processes acting inside them ? A new revolutionary method allows us to see beyond this opaque barrier: asteroseismology. Stars appear to vibrate like music instruments. Their vibration spectrum, their “music” is closely related to their internal structure. In the ASTA team, we model the vibrations of stars and develop methods at the front of present research allowing us to probe their internal structure based on the observed vibrations.
Convection and mixed modes in cold stars
The common character of "cold" stars such as our Sun and red giants is the presence of a convective envelope. The very turbulent convective motions near their surface excite their vibration modes like the fingers banging on a drum. According to our team's predictions, CoRoT and Kepler's most remarkable asteroseismic discovery was that of "mixed" modes in red giants. These modes have the unique ability to probe the dense core hidden under their giant envelope. They revealed a complete surprise: the core of red giants turns about 100 times slower than expected, raising the question: what is the mysterious process so efficiently slowing down their core? With the help of these mixed modes and the unique tools developed in our team to model the energetic interaction between convection and vibrations in these stars, we gradually unveil their core, for example when it is in the phase of carbon nucleosynthesis, the fundamental brick of life.
From stars to our galaxy
Thanks to CoRoT and Kepler observations (and from 2026 PLATO), we are able to measure with unequaled accuracy the distances, masses, radii and ages of tens of thousands of red giants in different regions of our Galaxy, as well as as the age of many white dwarfs. These measurements, complemented by the spectroscopic analysis of their chemical composition and the distances and radii coming from the GAIA space mission, constitute an essential database that opens the door to understanding the formation and evolution of our galaxy and the different populations of stars that compose it.
From stars to exoplanets
More and more exoplanets are discovered. The measurements of their masses, rays and ages pass through those of their host stars. By detecting their oscillations, our team is able to characterize them very precisely. The brown and red dwarfs, ultra-cold stars of very low mass, are the favorite target of the exoplanet hunters of the team of Mr. Gillon. We model them precisely to better characterize their planets.
Stars with mysterious origin
Some stars are really strange. This is the case of subdwarf B stars (sdB), also called extreme horizontal branch stars. These stars, in the process of synthesizing carbon from helium in their heart, have lost all their envelope probably because of the influence of a companion. How did it happen ? Luckily, these stars are vibrating too! Our team is at the forefront of probing their hearts with the techniques of asteroseismology and revealing their mysterious origin...
Helioseismology
Our Sun also vibrates. Thanks to its proximity, hundreds of thousands of modes are detected! They constitute a mine of information to probe its internal structure, this is the object of helioseismology. Our team also works hard in this domain. And we found new unexpected results. The hidden structure of the Sun is much more complex than expected, especially in a particular area called the tachocline, at the interface between its radiative core and its convective envelope.
