Binary formation through gas-assisted capture and the implications for stellar, planetary, and compact object evolution

Binary systems are ubiquitous and their formation requires two-body interaction and dissipation. In gaseous media, interactions between two initially unbound objects could result in gas-assisted binary formation, induced by a loss of kinetic energy to the ambient gas medium. Here, we derive analytically the criteria for gas-assisted binary capture through gas dynamical friction dissipation. We validate them with few-body simulations and explore this process in different gas-rich environments: gas-embedded star-forming regions (SFR), gas-enriched globular clusters, active galactic nucleus (AGN) discs, and protoplanetary discs. We find that gas-assisted binary capture is highly efficient in SFRs, potentially providing a main channel for the formation of binaries. It could also operate under certain conditions in gas-enriched globular clusters. Thin AGN discs could also provide a fertile ground for gas-assisted binary capture and in particular the formation of black hole/other compact object binaries, the production of gravitational-wave (GW) and other high-energy transients. Large-scale gaseous discs might be too thick to enable gas-assisted binary capture and previous estimates of the production of GW sources could be overestimated, and sensitive to specific conditions and the structure of the discs. In protoplanetary discs, while gas-assisted binary capture can produce binary Kuiper-belt objects, dynamical friction by small planetesimals is likely to be more efficient. Overall, we show that gas-assisted binary formation is robust and can contribute significantly to the binary formation rate in many environments. In fact, the gas-assisted binary capture rates are sufficiently high such that they will lead to multicaptures, and the formation of higher multiplicity systems.