The waves which propagate within stars, in this case the Sun, influence their structure, their dynamics and how they evolve. Since they can generate global pulsations, they provide us with precious information for studying dynamic phenomena within stars. There are two types of these waves:
1. acoustic waves, which are exactly the same as sound waves;
2. and gravity waves, which propagate in any density-stratified fluid, without large-scale (macroscopic) convective motion.
These waves play an important role in changes in the rotation and mixing of chemical elements within the deep layers of stars similar to our Sun. Like acoustic waves, gravity waves can, depending on their frequency, resonate within the Sun in what are known as "g modes": this produces the same phenomenon as that seen with a guitar string, which resonates according to certain harmonics and its own specific modes.
Thanks to the non-linear simulations carried out by the Astrophysics, Instrumentation and Modelling team as part of the ERC STARS 2 project, it is possible to compare the amplitude of these waves within the Sun's core and track their presence and their dynamics from core to surface. These results provide a solid base on which to refine theoretical models and open the way forward to more targeted observations, thus helping to develop increasingly more accurate knowledge of solar-like stars and how they evolve.
These simulations are not only unprecedented but also represent a genuine tour de force given the number of phenomena and the spatial scale taken into account: turbulence, convection, thermal, radiative and viscous effects, differential rotation, to name but a few. For the very first time, the results provide an extremely rich and comprehensive 3D simulation (97% of the Sun's volume) of stellar dynamics in three dimensions. From a purely numerical angle, these results are so impressive because of the ASH code, jointly developed and used by the team.
This involves high-performance simulations, requiring 5 million computing hours using the GENCI Ada computer, installed at the CNRS's Scientific Computing Resources and Development Institute (IDRIS), and 15 million hours using the Curie supercomputer made available by GENCI to the European researchers within the framework of PRACE, the Partnership for Advanced Computing in Europe.
The interior of a star consists of two zones, whose dimensions depend on the star's mass:
Furthermore, how the radiative zone and the convective zone are coupled together is one of the key questions in solar Physics today, one that is a major numerical challenge given the vast range of space-time scales implied. Indeed, there are numerous phenomena to be taken into account, and they do not all occur at the same scale, but each is influenced by the others in highly complex ways.
These results open up many new prospects for exploration, such as the potential to also factor in the magnetic field and its interaction with gravity waves, and thus come even closer to reality.