The early dynamical evolution of cool, clumpy star clusters

Abstract

Observations and theory both suggest that star clusters form sub-virial (cool) with highly sub-structured distributions. We perform a large ensemble of N-body simulations of moderate-sized (N=1000) cool, fractal clusters to investigate their early dynamical evolution. We find that cool, clumpy clusters dynamically mass segregate on a short timescale, that Trapezium-like massive higher-order multiples are commonly formed, and that massive stars are often ejected from clusters with velocities > 10 km/s (c.f. the average escape velocity of 2.5 km/s). The properties of clusters also change rapidly on very short timescales. Young clusters may also undergo core collapse events, in which a dense core containing massive stars is hardened due to energy losses to a halo of lower-mass stars. Such events can blow young clusters apart with no need for gas expulsion. The warmer and less substructured a cluster is initially, the less extreme its evolution.Comment: Accepted for publication in MNRAS; supplementary material can be downloaded at http://sgoodwin.staff.shef.ac.uk/all_plots.pdf.g

G0.253 + 0.016: A Molecular Cloud Progenitor of an Arches-Like Cluster

“…As such, its detailed study can reveal important clues about massive cluster formation and help test theoretical models. Current theories suggest that the early evolution of stellar clusters depends crucially on the spatial and kinematic distribution of stars (e.g., Goodwin & Bastian 2006;Allison et al 2010). However, while the distribution of emergent stellar populations as "hierarchical" or "centrally condensed" may be straightforward to quantify when analyzing stars, quantifying the small-scale distribution of the gas in a similar way is complicated (see, e.g., Lomax et al 2011).…”

Superbubble dynamics in globular cluster infancy

“…This might happen if a GC is formed from merging sub-clumps. In this case, strong mass segregation of the stars is expected to arise in about 10 6 yr (Allison et al 2010;Girichidis et al 2012;Pang et al 2013). Assuming a velocity dispersion inversely proportional to the stellar mass, as in Leigh et al (2013), would result in our relevant FRMS population having velocities of about the sound speed.…”

Role of Gas Dynamical Friction in the Evolution of Embedded Stellar Clusters

“…It is seen from numerical simulations that, clusters that are dynamically cool at birth can remain bound, down to SFE's ∼ 0.05, if they lose the gas while they are still cool (Goodwin 1997(Goodwin , 2009Smith et al. 2011) and that, clusters that are cool and/or have substructure, can mass segregate in a very short time scale (Allison et al 2009(Allison et al , 2010Maschberger & Clarke 2011;Olczak, Spurzem & Henning 2011). However, the study by Pelupessy & Portegies Zwart (2012), which extended these studies by including the potential due to the gas, found that, only a remnant with an IMF that was possibly skewed, rather than a mass segregated cluster, is left after gas expulsion.…”

“…Most stars seem to form in clusters, and hence, understanding clusters becomes an essential requirement for understanding the formation and evolution of galaxies. Two puzzles associated with open clusters that have attracted a lot of attention are -their formation, with densities and velocity dispersions that are not too different from those of the star forming regions in the Galaxy, given that the observed Star Formation Efficiencies (SFE) are low (Tutukov 1978;Hills 1980;Mathieu 1983;Elmegreen 1983;Lada, Margulis & Dearborn 1984;Adams 2000;Geyer & Burkert 2001;Baumgardt & Kroupa 2007;Goodwin 2009) and, the mass segregation observed / inferred in some of them (de Grijs et al 2002;Sirianni et al 2002;Gouliermis et al 2004;Chen, de Grijs & Zhao 2007;Converse & Stahler 2010;Hasan & Hasan 2011), at ages significantly less than their dynamical relaxation times (Bonnell & Davies 1998;McMillan, Vesperini & Portegies Zwart 2007;Allison et al 2009Allison et al , 2010Maschberger & Clarke 2011;Olczak, Spurzem & Henning 2011). In this paper, by making analytical approximations, we explore the efficacy of gas dynamical friction, operating during the embedded phase of stellar clusters, in solving these two puzzles.…”