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

Sambaran Banerjee; Pavel Kroupa

doi:10.1051/0004-6361/201526928

paper

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

arXiv:1510.04293 · doi:10.1051/0004-6361/201526928

Abstract

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. 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 ( $≳ 1 0^{4} M_{⊙}$ ), to become a young massive star cluster, as we observe today. To that end, model massive clusters (of initially $1 0^{4} M_{⊙} - 1 0^{5} M_{⊙}$ ) are evolved using Sverre Aarseth's state-of-the-art N-body code NBODY7. 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. 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-evolutionary mass loss, dynamical heating by primordial binaries and stellar-mass black holes, up to the observed sizes of young massive clusters; they always remain much more compact compared to the observed ones. This calls for additional mechanisms that can 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 $\approx 30$ % star formation efficiency, can indeed swell the newborn embedded cluster adequately. The limitations of the present calculations and their consequences are discussed.

20 pages, 9 figures (2 in colour), 4 tables. Accepted for publication in Astronomy and Astrophysics

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

Abstract

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