The possible role of stellar mergers for the formation of multiple stellar populations in globular clusters

Abstract

Many possible scenarios for the formation of multiple stellar populations (MSP) in globular clusters (GCs) have been discussed so far, including the involvement of asymptotic giant branch stars, fast rotating main sequence stars, very massive main sequence stars and mass-transferring massive binaries based on stellar evolution modelling. But self-consistent, dynamical simulations of very young GCs are usually not considered. In this work, we perform direct N -body modelling such systems with total masses up to 3.2×10 5 M , taking into account the observationally constrained primordial binary properties, and discuss the stellar-mergers driven both by binary stellar evolution and dynamical evolution of GCs. The occurrence of stellar mergers is enhanced significantly in binary-rich clusters such that stars forming from the gas polluted by mergers-driven ejection/winds would appears as MSPs. We thus emphasize that stellar mergers can be an important process that connects MSP formation with star cluster dynamics, and that multiple MSP formation channels can naturally work together. The scenario studied here, also in view of a possible top-heavy IMF, may be particularly relevant for explaining the high mass fraction of MSPs (the mass budget problem) and the absence of MSPs in young and low-mass star clusters.

BSE versus StarTrack: Implementations of new wind, remnant-formation, and natal-kick schemes in NBODY7 and their astrophysical consequences

“…In principle, in such an environment, a given massive star can undergo multiple mergers until a BH is formed (e.g. Fujii & Portegies Zwart 2013;Wang et al 2020) or even a BH non-progenitor can grow in mass to become a BH progenitor. Finally, we note that BH masses can get altered by not only such star-star mergers but also by other stellar-hydrodynamical processes such as envelope ejection during a CE phase and enhanced stellar wind in a tidally interacting binary; such processes can either be an outcome of massivebinary evolution or triggered dynamically (or both).…”

Linking globular cluster structural parameters and their evolution: multiple stellar populations

“…Alternative scenarios predict that the anomalous stars form as a consequence of processes that can only happen in primordial GCs (e.g. accretion of processed material on to the zero-age main-sequence stellar disc, formation and evolution of a central supermassive star; Bastian et al 2013;Gieles et al 2018;Wang et al 2020).…”

Different sodium enhancements among multiple populations of Milky Way globular clusters

“…This picture leads us to speculate about some kind of loss of strength or deceleration of the Na enrichment process inside the globular clusters. If the second generation of stars were the result of the interaction of binary stars, the ejecta of asymptotic giant branch stars, fast rotating main sequence stars, or stellar mergers, among others (Wang et al 2020), the above findings would imply that those processes would have lost effectiveness in producing Na-rich stars. The lack of detection of multiple populations in star clusters younger than 2 Gyr and less massive than ∼10 5 M (see Fig.…”

“…This is because it has been measured in every stellar aggregate harboring multiple populations, so that intrinsic Na spreads have been used as observational evidence. The mechanism that triggers the enhancement of Na, and light elements in general, is still under debate; a summary of the elements can be found in Wang et al (2020). The most frequently discussed scenarios suggest the existence of polluters inside the clusters, which are responsible for the chemical enhancement, namely: intermediate-mass asymptotic giant branch stars, fast rotating massive main sequence stars, interacting massive binaries, and super massive main sequence stars.…”