Between its formation stage as an active accreting seed and today, the Sun underwent large structural changes as well as variation in magnetic activity, rotation rate and its relation to the surrounding environment. I will go through the different processes that are responsible for these changes and present our latest results on the subject. We will go from the early interaction between the star and its proto-planetary disc bathing in UV radiations emitted from the massive neighbours, to the internal mixing happening in the inner layers of solar-like stars and probed by asteroseismology.
Finally, I will try to review the possible applications of this work to other types of stars at different stages of the evolution in the context of the current or future surveys.

The formation and evolution of light elements in the Universe act as important cosmological constraints. The oldest stars of the Galaxy have long been assumed to display in their outer layers the primordial lithium abundance, although all studies of stellar physics proved that this abundance must have decreased with time. The primordial Li abundance deduced from the observations of the Cosmological Background is indeed larger than the maximum one observed in these stars. Recent observations gave evidence of a large Li abundance dispersion in very metal poor stars.
During this presentation, we show that the amount of depletion needed to solve the lithium problem, can be accounted for a large part by the transport processes of chemical elements in stars. We show that stellar models including non-standard transport processes such as the rotation-induced mixing and penetrative convection are able to explain the lithium surface abundances of Population II stars, when using a lithium initial abundance in accordance with the primordial lithium abundance obtained from latest BBN results.
We also address the general question of the lithium abundance dispersion obtained from observations of carbon enhanced metal-poor stars, and how the interplay of atomic diffusion and accretion of matter modifies the element abundances in these metal-poor stars. In particular, we focus on the hydrodynamic processes that could take place after accretion. We show that the observations of lithium dispersion, associated with carbon enrichment, are well accounted for in terms of accretion onto the metal-poor stars, with accreted masses smaller than a few Jupiter masses, when using a primordial initial lithium abundance.

(Starts at 13:45 - a zoom session might be made available depending on technicalities).

The motion of three bodies can be solved perturbatively when a tightly bound inner binary is orbited by a distant perturber on an elliptic orbit. The exchange of angular momentum between the two orbits gives rise to Kozai-Lidov oscillations of eccentricity and inclination characterizing many different systems in astronomy. In this talk, I will first review the Newtonian version of the Kozai-Lidov mechanism before introducing the effect of relativistic corrections with applications to gravitational wave astronomy. I will show how one can systematically compute first post-Newtonian terms in the hierarchical three-body problem and discuss their influence on the dynamics of the system. Finally, I will present a new kind of relativistic resonance and its effect on the eccentricity of the inner binary, which could potentially be measurable in the phase of gravitational waves.

Ces dernières années l’astrophysique a connu des avancées décisives grace à la détection des ondes gravitationnelles et des neutrinos de haute énergie. Ces nouveaux messagers nous enseignent sur l’environnement des objets dits compacts autour lesquels la gravitation est extrême. Cette conférence dressera un tour d’horizon de nos connaissances dans les différents domaines de l’astrophysique en milieu extrême.

Microturbulence produced by plasma instabilities plays an important role in the dynamics and dissipation mechanisms of relativistic astrophysical collisionless shocks, such as those associated with gamma-ray bursts and blazar environments. We first present the tenets of an analytical model that describes the dynamics of the precursor of relativistic unmagnetized collisionless shock waves in electron-positron and electron-ion plasmas. The model is compared with results from particle-in-cell simulations and shown to accurately capture the deceleration and heating of the background plasma species. We further address the dynamics of relativistic radiation mediated shock waves that dictate the early emission in numerous transient sources such as supernovae and binary neutron star mergers. In the relativistic regime, a high pair multiplicity inside the shock transition leads to a lepton-baryon velocity separation, prone to plasma instabilities. We present a theoretical and numerical analysis of the hierarchy of plasma instabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Based on this analysis, we extend the analytical model introduced in the first part to demonstrate nonadiabatic compression in a Joule-like heating process by the joined contributions of the decelerating turbulence, radiation force, and electrostatic field. These results show that this analytical framework can be useful in describing how the microturbulence impact the dynamics and energy partition of a large class of relativistic shock waves.

Sujet : séminaire A.van Thieghem
Heure : 5 oct. 2021 02:00 PM Paris

Participer à la réunion Zoom
https://umontpellier-fr.zoom.us/j/97456080461

ID de réunion : 974 5608 0461

Streaming cosmic rays can power the exponential growth of a seed magnetic field by exciting a non-resonant instability that feeds on their bulk kinetic energy. In this work we investigate analytically and numerically the effects of the background plasma temperature and particle collisions on the instability. We find that increasing the temperature of the ambient plasma can substantially reduce the growth rate and the magnitude of the saturated magnetic field. In collisionless or poorly collisional plasmas, we demonstrate that the instability generates pressure anisotropies that act to reduce the magnetic field amplification. We show that when proton Coulomb collisions are sufficiently frequent, these pressure anisotropies are suppressed and the magnetic field energy density can be amplified to values 20% higher than in the collisionless case. In contrast, simulations of poorly ionized plasmas confirm the rapid damping of the instability with increasing proton-neutral collisions predicted by linear fluid theory calculations. In addition to astrophysical applications, these results pave the way for the design of laboratory experiments on the non-resonant streaming instability.

This talk is devoted to oscillations of thin accretion disks. After reviewing basic results of linear theory, we will introduce a new way how to examine disk oscillations in weakly nonlinear regime. The formalism will be applied to nonlinear evolution of the self-trapped g-mode, an interesting candidate for explanation of the fast X-ray quasi-periodic variability of accreting black-hole binaries. We will show that a natural consequence of this nonlinear evolution is excitation of two high-frequency p-waves emerging from the original g-mode domain whose frequencies are in 3:2 ratio.

Decisive advances in the understanding of explosive transients have been made possible by the development of multi-messenger and time-domain astronomy. However, the origins of high-energy (HE) to ultra-high-energy (UHE) cosmic rays and neutrinos remain unclear. Deciding between the various candidate sources requires a precise modeling of the propagation, acceleration and interactions of cosmic rays, and the subsequent multi-messenger and time-dependent emissions. In order to gain more insight into these physical processes, we study the production of HE-UHE cosmic rays and neutrinos in the vicinity of compact objects. We consider various categories of sources, and selected sources such as tidal disruption events, neutron star mergers or pulsar magnetospheres and winds. In addition, we assess the detectability of HE-UHE cosmic rays and neutrinos with future detectors such as GRAND and POEMMA.

Lien zoom: Sujet : Séminaire C. Guépin
Heure : 7 sept. 2021 02:00 PM Paris

Participer à la réunion Zoom
https://us02web.zoom.us/j/89543253080?pwd=NTBnSGJINUJmL0ljeDJ2NHdaOXBoQT09
ID de réunion : 895 4325 3080
Code secret : n49721