Seminars Temps-Espace-Société
Unless otherwise stated: Monday at 2 pm – Jean-François Denisse room/Observatoire de Paris, 77 Avenue Denfert-Rochereau, F-75014 PARIS
For people outside the Observatoire de Paris: please contact the organizers in advance.
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| After his groundbreaking achievements in establishing the special and the general theories of relativity in 1905 and 1915, Einstein pursued a program of finding a mathematical representation in which the gravitational and electromagnetic fields emerged as a unified entity. Physicists, historians, and philosophers have largely dismissed this endeavor as having been fruitless. Nevertheless, Einstein entertained high hopes in his search for a unified theory despite frequent setbacks and disappointments when he encountered difficulties and problems over and over again. Ironically, his search produced some insights that proved valid in astrophysical and cosmological contexts, while they were intended to advance a quantum theory of matter. In the presentation, I will take a look at Einstein's unified field theory program from a historical perspective of his overarching heuristics. |
For the last 60 years, theoretical models have struggled to explain the observed level of tidal dissipation in bodies with substantial convective envelopes, such as giant planets and solar type stars. In this talk, we review these models and show that they break down when tides are rapid, that is, when the tidal period is short compared to the convective turnover timescale. We propose a new formalism applicable in this regime. Assuming that tides transfer their energy to the convective flow, this formalism yields values for the tidal dissipation factor of Jupiter and Saturn, as well as for the circularization periods of solar-type binaries and extrasolar planets, in good overall agreement with observations. Lastly, we show that the phase lag and the tidal dissipation factor Q vary by orders of magnitude throughout a gaseous envelope, and we define average quantities that can be directly compared with observational data.
At the very end of Jupiter's growth, the planet was surrounded by a disk of gaseous refractory dust and ice, from which the Galilean moons are thought to have formed. However, the physical and chemical properties of this disk are not well constrained, and the formation of the Galilean moons remains a major theoretical challenge.
Our knowledge of the physical properties of the circumplanetary disk has improved over the last decade thanks to three-dimensional hydrodynamic models of the protoplanetary disk, in which a forming Jupiter is introduced. They show a very massive and hot disk, which could be rapidly depleted to a cold and light disk depending on the accreting flow properties. Although very accurate, these models could only simulate evolution over a few thousand years, whereas one needs to study the disk over 100 kyr and up to 1 Myr of evolution to study the formation of the moons.
My work has therefore focused on adapting simpler models to take account of the new clues to the physics of the circumplanetary disk provided by hydrodynamic simulations. In this seminar, I will present a two-dimensional model that accounts for the evolution of Jupiter's surface temperature, which produces the unexpected effect of the disk's self-shadow. This shadow cast on the disk creates a cold region where volatile ices can form close to Jupiter. I will discuss the possible implications of this effect on the composition of the disk and whether it can have an impact on the moon formation process.
We present a novel extension of Hamiltonian mechanics to non-conservative systems built upon the Schwinger-Keldysh-Galley double-variable formalism. We begin by outlining Galley's action principle for initial-value problems, and how it enables a description of non-conservative processes from a Lagrangian standpoint. We clarify important subtleties regarding boundary conditions, the emergence of the physical trajectory, and a class of gauge transformations on the canonical momenta. Next, from a Legendre transform, we construct the corresponding family of gauge-related nonconservative Hamiltonians. We show that virtually any classical initial-value problem can be embedded on this enlarged symplectic manifold, supplying the associated Hamiltonian and Lagrangian functions explicitly. As a further contribution, we derive a completely equivalent linear "Lie" formulation of the double-variable action and Hamiltonian which streamlines computations and renders transparent many structural properties of the formalism.
In this talk I will describe what is jet transport, also known as differential algebras, and describe some results we have obtained when applying these techniques in the initial orbit determination problem. After motivating their use and describing some details, I shall present our results on the determination of the transversal Yarkovsky parameter for Apophis. I will then consider the preliminary orbit determination problem, thus simulating the discovery of Near-Earth Objects, including some concrete examples as well as statistical results which show the quality of the results obtained. This is joint work with Luis Eduardo Ramírez Montoya and Jorge A. Pérez Hernández.
https://cnrs.zoom.us/j/91731122627?pwd=r8bH2boPa3ls6aJ5oklPiSinQKUrEe.1
Accurate 3D shape and rotation axis estimation of small celestial bodies are crucial for deep space missions like asteroid sample return and planetary defense, enabling navigation and risk assessment. However, highly variable observation conditions—from distant approach to close-range survey—pose significant challenges for any single modeling method. This work presents a synergistic multi-scale framework that integrates optimal modeling strategies for each observation phase: (1) At far range, a voxel-divided shape-from-silhouette (VD-SFS) method efficiently reconstructs initial shape and rotation axis from silhouette images. (2) In mid-range, a structure-from-motion (SfM) based approach dynamically estimates camera poses and generates global shape and axis models from sparse, feature-distinct images. (3) At close range, a photometry-based method exploits high-resolution, multi-angle images to recover local terrain at pixel-level detail by leveraging surface reflectance and illumination cues. This hierarchical, adaptive framework enables robust, coherent, and autonomous estimation of shape and rotation state throughout the full exploration sequence. Validation using simulated and real mission data (e.g., Rosetta, OSIRIS-REx) demonstrates that the proposed approach substantially improves autonomy, efficiency, and precision in small body exploration.
https://cnrs.zoom.us/j/94599131540?pwd=TaVwZzQGtCnPxNW7HwnyeeiUZApfbc.1