Planet-Driven Scatterings of Planetesimals Into a Star: Probability, Timescale and Applications

A planetary system can undergo multiple episodes of intense dynamical activities throughout its life, resulting in the production of star-grazing planetesimals (or exocomets) and pollution of the host star. Such activity is especially pronounced when giant planets interact with other small bodies during the system's evolution. However, due to the chaotic nature of the dynamics, it is difficult to determine the properties of the perturbing planet(s) from the observed planetesimal-disruption activities. In this study, we examine the outcomes of planetesimal-planet scatterings in a general setting, with the goal of determining the likelihood and timescale of planetesimal disruption by the host star as a function of the planet properties. We obtain a new analytical expression for the minimum distance a scattering body can reach, extending previous results by considering finite planet eccentricity and non-zero planetesimal mass. Through N-body simulations, we derive the distribution of minimum distances and the likelihood and timescales of three possible outcomes of planetesimal-planet scatterings: collision with the planet, ejection, and disruption by the star. We identify four defining dimensionless parameters (the planet eccentricity, planet-to-star mass ratio, planet radius to semi-major axis ratio, and the stellar disruption radius to planet semi-major axis ratio) that enable us to scale the problem and generalize our findings to a wide range of orbital configurations. Using these results, we explore three applications: falling evaporating bodies in the Beta Pictoris system, white dwarf pollution due to planetesimal disruption and planet engulfment by main-sequence stars.Comment: Submitted to MNRA

Experimental study of internal wave generation by convection in water

We experimentally investigate the dynamics of water cooled from below at 0^oC and heated from above. Taking advantage of the unusual property that water's density maximum is at about 4^oC, this set-up allows us to simulate in the laboratory a turbulent convective layer adjacent to a stably stratified layer, which is representative of atmospheric and stellar conditions. High precision temperature and velocity measurements are described, with a special focus on the convectively excited internal waves propagating in the stratified zone. Most of the convective energy is at low frequency, and corresponding waves are localized to the vicinity of the interface. However, we show that some energy radiates far from the interface, carried by shorter horizontal wavelength, higher frequency waves. Our data suggest that the internal wave field is passively excited by the convective fluctuations, and the wave propagation is correctly described by the dissipative linear wave theory

BEAST detection of a brown dwarf and a low-mass stellar companion around the young bright B star HIP 81208

Recent observations from B-star Exoplanet Abundance Study (BEAST) have illustrated the existence of sub-stellar companions around very massive stars. In this paper, we present the detection of two lower mass companions to a relatively nearby ($148.7^{+1.5}_{-1.3}$ pc), young ($17^{+3}_{-4}$ Myr), bright (V=$6.632\pm0.006$ mag), $2.58\pm0.06~ M_{\odot}$ B9V star HIP 81208 residing in the Sco-Cen association, using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large Telescope (VLT) in Chile. Analysis of the photometry obtained gives mass estimates of $67^{+6}_{-7}~M_J$ for the inner companion and $0.135^{+0.010}_{-0.013}~M_{\odot}$ for the outer companion, indicating the former to be most likely a brown dwarf and the latter to be a low-mass star. The system is compact but unusual, as the orbital planes of the two companions are likely close to orthogonal. The preliminary orbital solutions we derived for the system indicate that the star and the two companions are likely in a Kozai resonance, rendering the system dynamically very interesting for future studies.Comment: 18 pages, 14 figures, 5 tables Accepted for publication in the 10. Planets and planetary systems section of A&

Dynamical study of the exoplanets and debris disks revealed by SPHERE

Plusieurs décennies après l'identification des premiers disques de débris et des exoplanètes, les mécanismes de formation et d'évolution des systèmes planétaires sont encore loin d'être élucidés. Les récents progrès de l'imagerie directe à haute résolution et haut contraste, illustrés par les instruments VLT/SPHERE et Gemini/GPI, nous permettent désormais de révéler et d'étudier en détail l'architecture externe (> 5 ua) des systèmes extrasolaires jeunes ( 5 au) of young (< 200 Myr) extrasolar systems when the dynamical interactions are frequent. This work sheds light on the origin and dynamical evolution mechanisms of planetary systems through the detailed study of key systems resolved with SPHERE and through the developing of dedicated tools.The first part of this manuscript tackles the subject of N-body simulations. Numerous algorithms have been proposed and implemented, with different compromises on their speed, accuracy, and versatility. Among these algorithms, SWIFT HJS allows us to model for secular times architectures that are very different from our Solar System. It is thus an essential tool to the study of planetary to stellar companions with non-negligible mass ratio, which are often encountered with direct imaging. Within my Ph.D., the functionalities of the algorithm were extended to handle hierarchy changes and close encounters, which can play an important part in the dynamical history of planetary systems. The code was used to study in detail the mysterious system HD 106906, in particular, the interactions between its main components (binary star, planet, debris disk).In the second part of the manuscript, I introduce the subject of orbital fitting. The observation of a system at different epochs allows theoretically to retrieve the characteristics of the orbits. However, the problem is often complex and degenerate, in particular when the observations span a small fraction of the orbital period. The widely used MCMC statistical approach enables to get robust estimates in most of the cases. These estimates are then used to study the history and stability of the system, and the interactions between the orbits and their environment, notably the disks. This role of orbital fitting is here illustrated by the study of several benchmark systems imaged with SPHERE

Etude dynamique des exoplanètes et des disques de débris révélés par SPHERE

Several decades after the discovery of the first debris disks and exoplanets, lots of questions remain regarding the mechanisms of formation and evolution of planetary systems. The recent progress of high-resolution high-contrast direct imaging, illustrated by the instruments VLT/SPHERE and Gemini/GPI, enables to unveil the outer architecture (> 5 au) of young ( 5 ua) des systèmes extrasolaires jeunes (< 200 Myr), à un âge où les interactions dynamiques sont encore fréquentes. Mon travail de thèse apporte un éclairage sur l'origine et les mécanismes d'évolution dynamique des systèmes planétaires à travers l'étude détaillée de systèmes clefs résolus par SPHERE et le développement d'outils de modélisations dédiés.La première partie de ce manuscrit aborde l'étude dynamique via les simulations N-corps. De nombreux algorithmes ont été proposés et implémentés, avec des choix de compromis différents sur leur vitesse, leur précision et leur polyvalence. Parmi ces algorithmes, SWIFT HJS permet de modéliser des architectures très différentes de notre Système Solaire sur des temps séculaires. C'est donc un outil essentiel pour étudier l'influence des planètes massives, naines brunes et compagnons stellaires souvent rencontrés en imagerie directe. Durant ma thèse, les fonctionnalités de l'algorithme ont été étendues pour pouvoir modéliser les changements de hiérarchie et les rencontres proches, des aspects de la mécanique orbitale qui ont souvent un rôle crucial dans l'histoire dynamique des systèmes planétaires. Ce code a notamment été utilisé pour étudier en profondeur l'énigmatique système HD 106906 et les différentes interactions entre ses principaux composants (binaire, planète, disque de débris). Dans la deuxième partie du manuscrit, j'introduis la problématique de l'ajustement orbital. Si l'observation d'un système à différentes époques permet théoriquement de retrouver les caractéristiques de son orbite, le problème peut se révéler complexe et dégénéré, en particulier quand le temps d'observation est insuffisant pour correctement échantillonner l'orbite. L'approche statistique la plus couramment adoptée, le MCMC, permet d'obtenir des estimations fiables dans la plupart des cas. Ces estimations sont ensuite exploitées pour étudier l'histoire et la stabilité du système et les interactions entre les orbites et leur environnement, notamment les disques. Ce rôle de l'ajustement orbital est ici illustré dans les études de plusieurs systèmes de référence, imagés par SPHERE

Apsidal Alignment and Anti-Alignment of Planets in Mean-Motion Resonance: Disk-Driven Migration and Eccentricity Driving

Planets migrating in their natal discs can be captured into mean-motion resonance (MMR), in which the planets' periods are related by integer ratios. Recent observations indicate that planets in MMR can be either apsidally aligned or anti-aligned. How these different configurations arise is unclear. In this paper, we study the MMR capture process of migrating planets, focusing on the property of the apsidal angles of the captured planets. We show that the standard picture of MMR capture, in which the planets undergo convergent migration and experience eccentricity damping due to planet-disc interactions, always leads to apsidal anti-alignment of the captured planets. However, when the planets experience eccentricity driving from the disc, apsidally aligned configuration in MMR can be produced. In this configuration, both planets' resonance angles circulate, but a "mixed" resonance angle librates and traps the planets near the nominal resonance location. The MMR capture process in the presence of disc eccentricity driving is generally complex and irregular, and can lead to various outcomes, including apsidal alignment and anti-alignment, as well as the disruption of the resonance. We suggest that the two resonant planets in the K2-19 system, with their moderate eccentricities and aligned apsides, have experienced eccentricity driving from their natal disc in the past.Comment: 19 pages, 16 figure