Institute for Advanced Study Informal Astrophysics Seminar

Fluctuations in a stellar system's gravitational field cause the orbits of stars to evolve. These fluctuations can either originate from external perturbers (e.g. flybys), or from the intrinsic discreteness of the system (e.g. giant molecular clouds in a disc). Such a distinction allows us to address the pressing question of the respective roles of nature (system's internal properties) vs. nurture (cosmic environment) in the establishment of the observed properties of these systems. When accounting for finite-N effects, one may rely on the inhomogeneous Balescu-Lenard equation, to capture the induced orbital restructuration, and describe the associated secular diffusion. I will present this formalism, while emphasising in particular the key physical mechanisms at play in this context. When applied to a tepid stellar disc, it predicts the formation of narrow ridge-like structures in action space, in agreement with numerical simulations. In astrophysics, the inhomogeneous Balescu-Lenard equation is a new and rich framework, which may describe self-consistently the secular diffusion of giant molecular clouds in galactic discs, the thickening of stellar discs, or even the long-term evolution of population of stars within the Galactic centre, as I will show.