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.

Do All Stars in the Solar Neighbourhood Form in Clusters?

“…Since star/cluster formation happens in a dynamic environment [6,2,28] it may be impossible to define what will become a stellar cluster in these early stages, as the final object that remains bound is simply the dynamically mixed part of a larger, initially hierarchical, distribution. In this scenario a cluster can only be clearly defined above the surrounding distribution once it is dynamically evolved where t age /t cross > 1, as defined by Gieles & Portegies Zwart 2010.…”

Pre-Mainsequence Stellar Evolution in N-Body Models

“…The initial densities for the smallest clusters seem a little extreme but there is increasing evidence that the initial densities of open clusters are higher than previously thought (Parker et al 2009), that bound clusters can expand quickly (Bastian et al 2008;Moeckel & Bate 2010) and that rapid expansion can occur in the core (Kroupa, Aarseth, & Hurley 2001). Indeed Table 2 shows that the densest clusters expand the most because of the longer dynamical time required and, interestingly, end up with very similar half-mass radii to clusters that were initially somewhat sparser.…”

Formation of multiple populations in globular clusters: another possible scenario

“…the PS's potential well, the sizes of the stellar orbits increase (e.g., Moeckel & Bate 2010;Decressin et al 2010;Trenti et al 2010, and references therein), and in this way the massive PSs can also more easily lose their stars. If the gas expulsion is inefficient -for instance, when the massive PS has a lower interaction with larger structures -the PS does not lose a large number of stars because the gas has been retained, thus ending up as a dwarf galaxy.…”

“…after the expulsion of pristine gas, which leads to an expansion of the PS and a period of subsequent star loss (Moeckel & Bate 2010). Moreover, in a model where massive stars are considered to play an important role in the evolution of GCs, the formation of low-mass stars cannot be assumed to be instantaneous, because proto-low-mass stars have contraction times of several tens of Myr (Bernasconi & Maeder 1996), thus could become contaminated in that period of time by the winds of massive stars (Newsham & Terndrup 2007;Tsujimoto et al 2007).…”