From Few-Body to Stellar Clusters: How Gas Reshapes Dynamical Evolution

Gas-rich environments are ubiquitous in various scales, from protoplanetary disks to star clusters and galaxies. Dynamics in gas-rich environments are substantially different and give rise to unique astrophysical phenomena, along with enhancing the rates of well-studied phenomena. In this talk, I will outline some of my work on dynamics in gas-rich environments.

Nuclear star clusters (NSCs) are dense star clusters that reside at the centers of many galaxies. Due to their high densities, they serve as fertile grounds for various processes that are otherwise rare in less dense environments. The stellar distribution within NSCs is an important parameter in determining the rates of these events. NSCs can contain substantial amounts of gas, which reshape their stellar distributions and lead to steeper stellar cusps. I will re-derive and generalize the Bahcall-Wolf distribution to account for the effects of the added gas and discuss its implications for the rates of tidal disruption events (TDEs) and other transients in galactic nuclei.

Binaries in dense environments are traditionally classified as soft or hard based on their binding energy relative to the kinetic energy of surrounding stars. Heggie’s law suggests that stellar encounters tend to soften soft binaries and harden hard binaries, altering their separations. However, interactions with gas in such environments can significantly modify this behavior. I will discuss the impact of gas on binary softening and its consequences. I will show that gas interactions can actually harden binaries, extending the soft–hard boundary to larger separations. This introduces a “shielding radius” within which binaries are likely to harden due to gas interactions, surpassing the traditional soft–hard limit. Consequently, a notable portion of binaries initially classified as “soft” may become “hard” when both gas and stars are considered. While the exact criteria for common envelope (CE) formation are still uncertain, it is generally suggested that the binary pericenter should be small to initiate an unstable mass transfer. Hence, CE formation is expected to be more prevalent in gas-rich environments, including clusters hosting multiple populations, which are expected to be the majority of globular clusters. In these clusters, the existence of a second (or further) population of stars suggests a significant secondary epoch of gas, to support the formation of the second population of stars.

If time allows, I will discuss the von-Zeipel Lidov Kozai mechanism in gas-rich environments. All the binaries in active galactic nuclei (AGNs) are essentially triples with the central black hole as a third perturber. Also here, the interaction with gas could modify significantly the evolution and introduce new regimes of dynamical behavior.