Probing the formation of planetesimals in the Galactic Centre using Sgr A* flares

Flares in X-ray and near-infrared are observed above the quiescent emission of the supermassive black hole (SBH) in the Galactic Centre (GC) at a rate of approximately once per day. One proposed energy source for these flares is the tidal disruption of planetesimals with radius ≳10 km passing within ˜1 au of the SBH. Very little is known about the formation and evolution of planetesimals in galactic nuclei such as the GC, making predictions for flaring event rates uncertain. We explore two scenarios for the formation of planetesimals in the GC: (1) in a large-scale cloud bound to the SBH, and (2) in debris discs around stars. We model their orbital evolution around the SBH using the Fokker-Planck equation and investigate the effect of gravitational interactions with various relevant perturbers. Our predicted flaring rate, ≈0.6 d-1, is nearly independent of the distribution of perturbers. Moreover, it is insensitive to scenarios (1) or (2). The assumed number of planetesimals per star is consistent with debris discs around stars in the solar neighbourhood. In scenario (1), this implies that the number of planetesimals formed in the large-scale cloud is strongly correlated with the number of stars, and this requires finetuning for our results to be consistent with the observed flaring rate. We favour the alternative explanation that planetesimals in the GC are formed in debris discs around stars, similar to the solar neighbourhood.Computational astrophysicsStars and planetary system

White dwarf pollution by planets in stellar binaries

Computational astrophysic

Secular dynamics of hierarchical quadruple systems: the case of a triple system orbited by a fourth body

Computational astrophysicsStars and planetary system

Secular dynamics of multiplanet systems: implications for the formation of hot and warm Jupiters via high-eccentricity migration

Stars and planetary system

Chaos in self-gravitating many-body systems: Lyapunov time dependence of N and the influence of general relativity

Computational astrophysic

The Development of Language Learning Strategies

This article discusses the strategy repertoires and strategy development of six English children who learned foreign languages at primary school. My study differs from mainstream research in that it focuses on young children and on the development of their strategies, draws on sociocultural theory and uses ethnographic methods. My findings show that the six children developed a range of strategies over the course of a calendar year in spite of receiving no direct strategy instruction. The primary classroom encouraged learner autonomy and stimulated children to reflect on their learning which, in turn, enabled them to refine their strategies

Hierarchical systems

  The thesis addresses the long-term dynamical evolution of hierarchical multiple systems. First, we consider the evolution of orbits of stars orbiting a supermassive black hole (SBH). We study the long-term evolution and compute tidal disruption rates of stars by the SBH. Such disruption events reveal the physics and properties of stars and SBHs. Furthermore, we study the dynamics of planetesimals in the galactic center (GC). When planetesimals are tidally disrupted by the SBH, this can produce a potentially observable flare. We compute the rates of such disruptions, and find rates consistent with observations, suggesting that planetesimals are formed in the GC around stars, similarly to stars in the solar neighbourhood. Subsequently, we consider the long-term evolution of hierarchical quadruple systems. We appy our techniques to provide an explanation for the lack of transiting circumbinary planets around short-period binaries. Lastly, we generalise our methods, and apply them to study the implications of the long-term dynamical evolution of multiplanet systems on hot Jupiters (HJs). We find that the long-term dynamical evolution in multiplanet systems can explain at most a few per cent of the observed HJs, unless the efficiency of tidal dissipation is much higher than is currently believed.  </div

Programmation et validation d'un code d'elements finis 2D quadrangulaires pour une formulation lagrangienne des equations de la dynamique des gaz compressibles en regime instationnaire, avec implantation d'un remaillage d'ordre 2

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc