Abstract
The fraction of star formation that results in bound star clusters is influenced by the density spectrum in which stars are formed and by the response of the stellar structure to gas expulsion. We analyse hydrodynamical simulations of turbulent fragmentation in star-forming regions to assess the dynamical properties of the resulting population of stars and (sub)clusters. Stellar subclusters are identified using a minimum spanning tree algorithm. When considering only the gravitational potential of the stars and ignoring the gas, we find that the identified subclusters are close to virial equilibrium (the typical virial ratio Q_vir~0.59, where virial equilibrium would be Q_vir~0.5). This virial state is a consequence of the low gas fractions within the subclusters, caused by the accretion of gas onto the stars and the accretion-induced shrinkage of the subclusters. Because the subclusters are gas-poor, up to a length scale of 0.1-0.2 pc at the end of the simulation, they are only weakly affected by gas expulsion. The fraction of subclusters that reaches the high density required to evolve to a gas-poor state increases with the density of the star-forming region. We extend this argument to star cluster scales, and suggest that the absence of gas indicates that the early disruption of star clusters due to gas expulsion (infant mortality) plays a smaller role than anticipated, and is potentially restricted to star-forming regions with low ambient gas densities. We propose that in dense star-forming regions, the tidal shocking of young star clusters by the surrounding gas clouds could be responsible for the early disruption. This `cruel cradle effect' would work in addition to disruption by gas expulsion. We suggest possible methods to quantify the relative contributions of both mechanisms.
Highlights
Over the past years, the implications of clustered star formation have touched a range of astrophysical disciplines, from the scales of the star formation process itself to the fundamental properties of young star clusters (e.g. McMillan et al 2007; Allison et al 2009a; Moeckel & Bonnell 2009), or possibly even the global stellar mass assembly of galaxies
We have assessed the dynamical state of stellar structure in star-forming regions and its response to gas expulsion by analysing the properties of the stellar structure in the smoothed particle hydrodynamics (SPH)/sink particle simulations of Bonnell et al (2003, 2008)
The high-density end of the distribution is occupied by sink particles belonging to the subclusters that are identified with the MST. (2) When excluding the potential of the gas from the dynamical analysis and only considering the sink particles, we find that the simulation as a whole becomes marginally bound after one freefall time, and the population of individual subclusters is close to virial equilibrium
Summary
The implications of clustered star formation have touched a range of astrophysical disciplines, from the scales of the star formation process itself (see the review by McKee & Ostriker 2007) to the fundamental properties of young star clusters (e.g. McMillan et al 2007; Allison et al 2009a; Moeckel & Bonnell 2009), or possibly even the global stellar mass assembly of galaxies (see e.g. Pflamm-Altenburg et al 2007; Bastian et al 2010). The response of star clusters to gas expulsion has been investigated by Moeckel & Bate (2010), who consider N -body simulations of star clusters using initial conditions from hydrodynamic simulations, and by Moeckel & Clarke (2011), who address the dynamical evolution of star clusters under the condition of ongoing gas accretion. They propose that the disruptive effect of gas expulsion is limited by the way in which gas and stars are redistributed by the accretion-induced shrinkage of clusters. The paper is concluded with a summary and an outlook in Sect. 5, where we discuss the possible dependence of the results on the initial conditions of the simulations and the input physics, and suggest ways in which our analysis could be improved and extended
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