How can young massive clusters reach their present-day sizes?

Abstract

Context. The classic question that how young massive star clusters attain their shapes and sizes, as we find them today, remains to be a challenge. Both observational and computational studies of star-forming massive molecular gas clouds infer that massive cluster formation is primarily triggered along the small-scale ( 0.3 pc) filamentary substructures within the clouds. Aims. The present study is intended to investigate the possible ways in which a filament-like-compact, massive star cluster (effective radius 0.1-0.3 pc) can expand 10 times, still remaining massive enough ( 10 4 M ⊙ ), to become a young massive star cluster, as we observe today. Methods. To that end, model massive clusters (of initially 10 4 M ⊙ − 10 5 M ⊙ ) are evolved using Sverre Aarseth's state-of-the-art N-body code NBODY7. Apart from the accurate calculation of two-body relaxation of the constituent stars, these evolutionary models take into account stellar-evolutionary mass loss and dynamical energy injection, due to massive, tight primordial binaries and stellar-remnant black holes and neutron stars. These calculations also include a solar-neighbourhood-like external tidal field. All the computed clusters expand with time, whose sizes (effective radii) are compared with those observed for young massive clusters, of age 100 Myr, in the Milky Way and other nearby galaxies. Results. It is found that beginning from the above compact sizes, a star cluster cannot expand by its own, i.e., due to two-body relaxation, stellar mass loss, dynamical heating by primordial binaries and compact stars, up to the observed sizes of young massive clusters; they always remain much more compact compared to the observed ones. Conclusions. This calls for additional mechanisms that boost the expansion of a massive cluster after its assembly. Using further N-body calculations, it is shown that a substantial residual gas expulsion, with ≈ 30% star formation efficiency, can indeed swell the newborn embedded cluster adequately. The limitations of the present calculations and their consequences are discussed.

Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation

“…All the computed models are initially Plummer clusters with masses M cl (0) ≤ 5 × 10 4 M and half-mass radius r h (0) = 2 pc; r h (0) = 1 pc are also used for a few lower mass models. Such masses and sizes are typical for Galactic and Local Group YMCs (Banerjee & Kroupa 2016;Portegies Zwart et al 2010;Ryon et al 2015); considering the masses of GCs, they represent intermediatemass and open stellar clusters (see Sec. 1).…”

Searching for Be Stars in the Open Clusters with PTF/iPTF. I. Cluster Sample and Be Star Candidates

“…Many Herbig Ae/Be stars are found in this young massive cluster. Banerjee & Kroupa (2017) used N-body simulations to study the cluster size of young massive clusters. They found that the simulated results were much more compact than observed one.…”

High angular resolution study of the super star cluster population in IRAS 17138−1017

“…Apart from a smooth distribution, 50% of the dust is distributed fractally. We adopted N = 32, D = 2.6, H = 5 for the hierarchically clumpy distribution, a standard dust-to-gas ratio of 0.01 as suggested by Whelan et al (2011), and a representative star-forming efficiency (SFE) of 33% following Banerjee & Kroupa (2017). The SFE sets the mass of residual ISM and thus of dust, after stars have been formed.…”