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Stars: dynamical Processes driving tidal Interactions, Rotation and Evolution

Periodic Reporting for period 3 - SPIRE (Stars: dynamical Processes driving tidal Interactions, Rotation and Evolution)

Reporting period: 2018-09-01 to 2020-02-29

Stars are dynamical objects: they are rotating, magnetic, turbulent, pulsating, and they strongly impact their environment because of their winds and tides. This dynamics drastically affects their evolution, the evolution of their surrounding planetary systems and the conditions for the emergence of life, and galactic evolution. In this context, our knowledge of stellar interiors, rotation, and magnetism has undergone a revolution thanks to helio- and astero-seismology (with SOHO, CoRoT and Kepler/K2) and ground-based spectropolarimetry (ESPaDOnS/CFHT, Narval/TBL, HARPSpol/ESO). For example, helio- and asteroseismology revealed, e.g. that the core of the Sun is close to a uniform rotation until 20% of its radius while those of subgiant and red giant stars slow down drastically during their evolution. These important results demonstrate that powerful dynamical mechanisms (internal waves, turbulence, magnetic fields) are in action to extract angular momentum all along the evolution of stars.

Simultaneously, since the discovery of the first extrasolar planetary system in 1995 by Mayor & Queloz, a large diversity of planetary systems orbiting various types of stars has been discovered both from the ground (e.g. HARPS and WASP surveys) and from space (CoRoT, Kepler/K2) bringing the problematic of star-planet interactions on the forefront of academic research. Simultaneously, a large population of multiple stars has been discovered that provide key constraints on the structure and the evolution of stars. These groundbreaking discoveries have opened a golden age for the integrated study of the dynamics of stars and their environment with the selected/forthcoming new space and ground-based facilities among which CHEOPS (ESA), TESS (NASA), SPIRou (CFHT), JWST (NASA), PLATO (ESA), ARIEL (ESA), and E-ELT (ESO). It put the urgency to progress on our understanding of star-planet and star-star interactions: highly complex dynamical processes leading to tidal dissipation in stars and planets play a key role to shape the orbital architecture of planetary and stellar systems and they may deeply modify their evolution.

To interpret these observational breakthroughs, it is necessary to develop now new frontier theoretical and numerical long-term evolution models of rotating magnetic stars and of their systems. The key questions to address are:
- For a given evolutionary stage, which dynamical processes drive the rotational evolution of stars?
- How does this dynamical evolution impact their chemical properties and magnetic activity?
- How does tidal interactions vary as a function of stellar mass and evolutionary stage?
- What is the respective strength and impact of tidal and magnetic interactions in stellar systems along the evolution of their host star(s)?
- How do the orbital architecture (and habitable zone) change along the host star’s evolution?
- How do the evolution of stars with companions differ from those of single stars?

To reach this ambitious objective, the SPIRE project develop new groundbreaking equations, prescriptions, and scaling laws that describe coherently all dynamical mechanisms that transport angular momentum (and chemicals) in stars and drive tidal dissipation in stellar and planetary interiors using advanced semi-analytical modeling and numerical simulations. They will be implemented in the new generation dynamical stellar evolution code STAREVOL and secular orbital/rotational evolution code ESPEM (Planetary System Evolution and Magnetism). This will allow us to provide state-of-the-art ab-initio integrated and coupled models for the long-term evolution of stars and of their systems, which cannot be directly simulated in 3D yet.
SPIRE will thus provide key inputs for the whole astrophysical community: understanding the dynamics of stars is a fundamental step to understand our Universe.
The SPIRE project aims to understand the dynamical processes driving the rotational evolution of stars and the dynamics of the orbital architecture of stellar/planetary systems along the evolution of the host star(s).
To reach these objectives, it is structured around two tasks:
- Task I (TI) devoted to the dynamical evolution of rotating magnetic stars;
- Task II (TII) devoted to the study of tides and integrated evolution of stellar systems.

TI: Dynamical evolution of rotating magnetic stars

We are studying in a systematic way the physical mechanisms able to transport angular momentum in convectively stable stellar radiation zones that drive the secular rotational and chemical evolutions of stars. These mechanisms strongly impact the magnetism of stars and their interactions with their environment studied in Task II.

First, we have built new 2D local and global models for the transport of angular momentum by waves driven by buoyancy and the rotation of the star through the Coriolis acceleration, the so-called gravito-inertial waves. We take into account simultaneously buoyancy, all the components of the Coriolis and centrifugal accelerations, and radial and latitudinal differential rotation. The models allow us for the first time to treat every possible hierarchy of these four different processes without any a-priori. We have computed the properties of the waves propagation and dissipation and related prescriptions for the transport of angular momentum, heat and chemicals they induce. In addition, these models also allow us to develop new seismic diagnosis for rapidly, potentially differentially, rotating stars to get important constrains on their rotation profile and chemical stratification.
We are now working on the effects of general axi- and non-axisymmetric magnetic fields on stellar waves propagation, dissipation, and induced transport of angular momentum. This constitutes one of the key objectives of the second period of the project.

To properly evaluate the amplitude of the transport of angular momentum by waves in stellar radiation zones, we have to compute the strength of their excitation by adjacent convective regions. To reach this goal, we are first developing new 3D nonlinear global numerical simulations of the stochastic excitation of gravito-inertial waves by turbulent convective cores. This will allow us to get mandatory information on their amplitude and on their frequency spectrum as a function of key stellar parameters such as rotation. In addition, we have developed a new mixing-length theory for turbulent convection in rotating stars where we have taken into account both viscous and heat diffusions. It allows us to evaluate the depth of convective penetration (i.e. of undershooting below a convective envelope and of overshooting at the top of a convective core) and the variation of the amplitude of stochastically excited gravito-inertial waves as a function of stellar rotation. These results are of crucial importance since the rotation of stars strongly varies all along their evolution while its impact on convection and waves is completely ignored in current stellar evolution models.

Next, we have established a new theoretical framework to describe shear-induced turbulent transport in stellar radiation zones. Because of the combined action of stable stratification and rotation, this transport is highly anisotropic: the restoring forces for vertical and horizontal turbulent motions are buoyancy and the Coriolis acceleration, respectively. For the first time, we derive a prescription for the anisotropy of the turbulent transport of momentum and chemicals taking into account coherently the simultaneous action of these two forces. This allows us to compute a new turbulent transport coefficient that models the transport by turbulent motions generated by the instability of a vertical shear in the horizontal directions. This new theoretical prescription has been implemented in the dynamical stellar evo
In each task, our objective is to achieve the best semi-analytical and numerical modeling of dynamical processes in stellar interiors that impact the evolution of stars and of their surrounding systems on secular timescales. However, it is not yet possible to compute directly this long-term evolution in 3D, taking all the needed space scales related to often highly non-axisymmetric and non-linear short timescale MHD phenomena into account. We thus model the action of each dynamical process and of its couplings developing new advanced semi-analytical methods and high-resolution numerical simulations to derive cutting-edge original equations, prescriptions, and scaling laws for secular timescales. They are implemented in the dynamical stellar evolution code STAREVOL and the secular orbital/rotational evolution code ESPEM we are developing and that are used to model observed stars and their systems, leading to a direct confrontation of the predictions of our modeling with the observables (e.g. the differential rotation in stars obtained thanks to asteroseismology, the orbital properties of star-planet and multiple star systems, the relative inclination of stellar spins and the orbits, etc.).

By construction, the SPIRE project builds the needed completely new generation of dynamical models of stars and their environment taking into account coherently the impact of MHD transport and tidal dissipation mechanisms in stellar (and planetary) interiors on their long-term evolution. Therefore, SPIRE has a strong impact on stellar physics (for single and multiple stars) and provides key groundbreaking results, for instance the value of the tidal dissipation in stars of different masses and evolutionary stages, for the studies of star-planet interactions. It also strongly contributes to change the paradigm of the study of stellar systems where the host star’s evolution and the orbital architecture dynamics have been treated separately until now. In this context, it benefits from the best observational constraints obtained by the most advanced space missions and ground-based instrumentation devoted to explore stellar systems all along the project (K2, and soon CHEOPS, TESS, SPIRou, JWST). The SPIRE project also contributes significantly to the scientific preparation of PLATO for the modeling of single and multiple stars and star-planet interactions. SPIRE thus proposes and provides a new framework for the interpretation of current and forthcoming observational programs and actively contributes to develop and put its topics in the forefront of academic research.
Because of the different physical processes it studies, the SPIRE project also contributes to the state-of-the-art in astrophysical (and geophysical) fluid dynamics, in fundamental fluid mechanics, in plasma physics and in numerical astrophysics.

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

In this framework, it participates to the efforts to strengthen the European position in this research field by training a new generation of stellar and planetary physicists working on the dynamics of stars, of exoplanets, and of their environment, which constitute an essential research field of modern Astronomy and Astrophysics.
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