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Stars: dynamical Processes driving tidal Interactions, Rotation and Evolution

Periodic Reporting for period 4 - SPIRE (Stars: dynamical Processes driving tidal Interactions, Rotation and Evolution)

Reporting period: 2020-03-01 to 2021-02-28

Stars are rotating, magnetic, turbulent, pulsating objects. They strongly impact their environment because of their winds and tidal and magnetic star-planet interactions. This dynamics drastically affects the evolution of stars and their planets. Our knowledge of stellar interiors, rotation, and magnetism is undergoing a revolution thanks to (helio-) astero-seismology (e.g. Kepler/K2, TESS) and ground-based spectropolarimetry (e.g. ESPaDOnS & SPIRou/CFHT). For instance, seismology has revealed that powerful MHD mechanisms, which should be identified, are in action to strongly extract angular momentum from the core of stars all along their evolution.
Simultaneously, a large diversity of planetary systems orbiting various types of stars with various orbital architectures has been discovered from the ground and space. This brings star-planet interactions on the forefront and opens a golden age for the integrated study of the dynamics of stars and their environment with in a near future JWST, PLATO, ARIEL, and the E-ELT. It put the urgency to progress on our understanding of tidal interactions that shape planetary systems.

To interpret these observational breakthroughs, it is necessary to develop new frontier coupled secular evolution models of rotating magnetic stars and of their systems. To reach this ambitious objective, the SPIRE project has developed new innovative equations, prescriptions, and scaling laws that describe coherently dynamical mechanisms that transport angular momentum in stars and drive tidal dissipation in stellar and planetary interiors using advanced theory and numerical simulations. They have been implemented in the coupled new generation dynamical stellar evolution code STAREVOL and secular star-planet evolution code ESPEM (Planetary System Evolution and Magnetism). This allows us to provide state-of-the-art ab-initio models for the long-term evolution of stellar systems, which cannot be directly simulated in 3D yet.
To reach these objectives, the project is structured around 2 tasks:
- Task I (TI): dynamical evolution of rotating magnetic stars;
- Task II (TII): tides and integrated evolution of stellar systems.

In TI, we have studied in a systematic way the physical mechanisms able to transport angular momentum in stellar radiation zones that drive the secular rotational and chemical evolutions of stars. They strongly impact the magnetism of stars and their interactions with their environment studied in TII.
First, we have done a complete study of the excitation, the propagation and the dissipation of gravito-inertial waves that propagate in stellar radiation zones because of buoyancy and rotation. We demonstrate that their stochastic convective excitation can be amplified in rapidly rotating stars and we explore their propagation for any stratification and rotation. We show that they trigger an important angular momentum transport that deeply modifies the evolution of stars. Next, we have studied the impact of rotation and viscous and heat diffusions on stellar convection. We show how rotation strongly impacts penetrative convection and thus chemical mixing. Third, we provide the first complete study of the anisotropic turbulent transport sustained by shear-induced instabilities in stellar radiation zones. For the first time we take into account coherently stable stratification, (differential) rotation and heat diffusion. We improve our understanding of star’s magnetism along their evolution thanks to new scaling laws for the amplitude of magnetic fields generated by dynamo action and their potential remnant fossil fields. Finally, we have developed a new generation of seismic diagnosis, which take into account (differential) rotation and realistic 3D magnetic topologies; they open a new window for constraining internal rotation, magnetism, and mixing in stars. New theoretical prescriptions and numerical scaling laws have been implemented in the code STAREVOL.

In TII, we have studied in a systematic way the variation of the dissipation of tidal inertial/gravity waves propagating in the convective envelope/radiative core of low-mass stars along their evolution. We have demonstrated that it varies over several orders of magnitude with stellar mass, age, metallicity, and rotation. The dissipation of tidal gravity waves dominates at the beginning of the pre-main-sequence and after the end of the main sequence; otherwise, the dissipation of tidal inertial modes dominates. Next, we have examined the complex interactions of tidal waves with differential rotation, rotating turbulent convection and magnetic fields.
In addition, we have studied tidal dissipation in every planetary type. First, we demonstrated how the mixing of heavy elements in the envelope of gaseous giant planets could enhance the dissipation. Next, we study the dissipation of atmospheric and oceanic tides in telluric planets. For instance, we demonstrated how the dissipation of atmospheric tides in convective layers near the ground leads to asynchronous planetary rotation. Using these results we have actively developed the ESPEM code, which takes into account tides, stellar winds, and MHD Star-Planet Interactions. We predict that the evolution of hot-Jupiter systems is driven by tidal interactions while the evolution of a super-Earth orbiting a M-dwarf is driven by MHD torques.
In each task, our objective is to achieve the best semi-analytical and numerical modelling of dynamical processes that drive the secular evolution of stars and of their systems. However, it is not yet possible to compute this long-term evolution in 3D, taking into account all the needed space and time scales. We thus model the action of each mechanism developing new advanced semi-analytical methods and high-resolution numerical simulations to derive cutting-edge ab-initio equations, prescriptions, and scaling laws for the secular STAREVOL and ESPEM codes.

Therefore, SPIRE has a strong impact on stellar physics and has provided numerous groundbreaking results. It strongly contributes to change the paradigm of the study of planetary systems where the host star’s evolution and the orbital dynamics have been treated separately. It also contributes to the state-of-the-art in astrophysical/ geophysical fluid dynamics.

It addresses key societal questions on the impact of the dynamics of stellar/planetary systems on habitability and the conditions for the emergence of life. This provides key assets to understand the properties and history of our Sun-Earth and solar systems in the broader context provided by exoplanets.

It has contributed to the efforts to strengthen the European position in this research field by training a new generation of stellar and planetary physicists (3 members of the project got permanent positions; 3 prizes have been obtained) working on the dynamics of stars, of exoplanets, and of their environment.
The results obtained within the project are disseminated through peer-reviewed publications in international scientific journals, participation to international scientific conferences, lectures given by the members of the project in Universities and in Engineering schools, and public outreach.
Synopsis figure summarizing the SPIRE project