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Magnetic field amplification during gravitational collapse – influence of turbulence, rotation and gravitational compression

Authors

  • Sharanya Sur,

    Corresponding author
    1. Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
    2. Raman Research Institute, C. V. Raman Avenue, Sadashivnagar, Bangalore 560080, India
    3. Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411007, India
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  • Christoph Federrath,

    1. Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
    2. Monash Center for Astrophysics (MoCA), School of Mathematical Sciences, Monash University, Vic 3800, Australia
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  • Dominik R. G. Schleicher,

    1. Georg-August-Universität, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
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  • Robi Banerjee,

    1. Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
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  • Ralf S. Klessen

    1. Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
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E-mail: ssur06@gmail.com

ABSTRACT

We study the influence of initial conditions on the magnetic field amplification during the collapse of a magnetized gas cloud. We focus on the dependence of the growth and saturation level of the dynamo-generated field on the turbulent properties of the collapsing cloud. In particular, we explore the effect of varying the initial strength and injection scale of turbulence and the initial uniform rotation of the collapsing magnetized cloud. In order to follow the evolution of the magnetic field in both the kinematic and the non-linear regime, we choose an initial field strength of inline image with the magnetic to kinetic energy ratio, Em/Ek∼ 10−4. Both gravitational compression and the small-scale dynamo initially amplify the magnetic field. Further into the evolution, the dynamo-generated magnetic field saturates but the total magnetic field continues to grow because of compression. The saturation of the small-scale dynamo is marked by a change in the slope of B2/3 and by a shift in the peak of the magnetic energy spectrum from small scales to larger scales. For the range of initial Mach numbers explored in this study, the dynamo growth rate increases as the Mach number increases from vrms/cs∼ 0.2 to 0.4 and then starts decreasing from vrms/cs∼ 1.0. We obtain saturation values of Em/Ek= 0.2–0.3 for these runs. Simulations with different initial injection scales of turbulence also show saturation at similar levels. For runs with different initial rotation of the cloud, the magnetic energy saturates at Em/Ek∼ 0.2–0.4 of the equipartition value. The overall saturation level of the magnetic energy, obtained by varying the initial conditions, is in agreement with previous analytical and numerical studies of small-scale dynamo action where turbulence is driven by an external forcing instead of gravitational collapse.

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