On the evolution of a star cluster and its multiple stellar systems following gas dispersal

Abstract

We investigate the evolution, following gas dispersal, of a star cluster produced from a hydrodynamical calculation of the collapse and fragmentation of a turbulent molecular cloud. We find that when the gas, initially comprising ≈60 per cent of the mass, is removed, the system settles into a bound cluster containing ≈30–40 per cent of the stellar mass surrounding by an expanding halo of ejected stars. The bound cluster expands from an initial radius of <0.05 to 1–2 pc over ≈4–10 Myr, depending on how quickly the gas is removed, implying that stellar clusters may begin with far higher stellar densities than usually assumed. With rapid gas dispersal, the most massive stars are found to be mass segregated for the first ∼1 Myr of evolution, but classical mass segregation only develops for cases with long gas removal time-scales. Eventually, many of the most massive stars are expelled from the bound cluster. Despite the high initial stellar density and the extensive dynamical evolution of the system, we find that the stellar multiplicity is almost constant during the 10 Myr of evolution. This is because the primordial multiple systems are formed in a clustered environment and, thus, by their nature are already resistant to further evolution. The majority of multiple system evolution is confined to the decay of high-order systems (particularly quadruple systems) and the formation of a significant population of very wide (104–105 au) multiple systems in the expanding halo. This formation mechanism for wide binaries potentially solves the problem of how most stars apparently form in clusters and yet a substantial population of wide binaries exist in the field. We also find that many of these wide binaries and the binaries produced by the decay of high-order multiple systems have unequal mass components, potentially solving the problem that hydrodynamical simulations of star formation are found to underproduce unequal mass solar-type binaries.