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The dynamical state of stellar structure in star-forming regions

Authors

  • J. M. Diederik Kruijssen,

    Corresponding author
    1. Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, the Netherlands
    2. Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands
    3. Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
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  • Thomas Maschberger,

    1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
    2. Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
    3. UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d‘Astrophysique de Grenoble, UMR 5274, Grenoble 38041, France
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  • Nickolas Moeckel,

    1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
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  • Cathie J. Clarke,

    1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
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  • Nate Bastian,

    1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
    2. Excellence Cluster Universe, Boltzmannstraße 2, 85748 Garching, Germany
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  • Ian A. Bonnell

    1. Scottish Universities Physics Alliance (SUPA), School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS
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E-mail: kruijssen@mpa-garching.mpg.de

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 Qvir≈ 0.59, where virial equilibrium would be Qvir∼ 0.5). This virial state is a consequence of the low gas fractions within the subclusters, caused by the accretion of gas on to 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.

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