Compression of turbulent magnetized gas in giant molecular clouds

Yuval Birnboim*, Christoph Federrath, Mark Krumholz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


Interstellar gas clouds are often both highly magnetized and supersonically turbulent, with velocity dispersions set by a competition between driving and dissipation. This balance has been studied extensively in the context of gases with constant mean density. However, many astrophysical systems are contracting under the influence of external pressure or gravity, and the balance between driving and dissipation in a contracting, magnetized medium has yet to be studied. In this paper, we present three-dimensional magnetohydrodynamic simulations of compression in a turbulent, magnetized medium that resembles the physical conditions inside molecular clouds. We find that in some circumstances the combination of compression and magnetic fields leads to a rate of turbulent dissipation far less than that observed in nonmagnetized gas, or in non-compressing magnetized gas. As a result, a compressing, magnetized gas reaches an equilibrium velocity dispersion much greater than would be expected for either the hydrodynamic or the non-compressing case. We use the simulation results to construct an analytic model that gives an effective equation of state for a coarse-grained parcel of the gas, in the form of an ideal equation of state with a polytropic index that depends on the dissipation and energy transfer rates between the magnetic and turbulent components. We argue that the reduced dissipation rate and larger equilibrium velocity dispersion has important implications for the driving and maintenance of turbulence in molecular clouds and for the rates of chemical and radiative processes that are sensitive to shocks and dissipation.

Original languageAmerican English
Pages (from-to)2144-2159
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Jan 2018

Bibliographical note

Funding Information:
resources provided by the Leibniz Rechenzentrum and the Gauss Centre for Supercomputing (grants pr32lo, pr48pi and GCS Large-scale project 10391), the Partnership for Advanced Computing in Europe (PRACE grant pr89mu), the Australian National Computational Infrastructure (grant ek9), and the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia, in the framework of the National Computational Merit Allocation Scheme and the ANU Allocation Scheme. The simulation software FLASH was in part developed by the DOE-supported Flash Center for Computational Science at the University of Chicago.

Funding Information:
We thank Chalence Safranek-Shrader and Romain Teyssier for their help during the implementation of the Hubble source terms for MHD (Section 2). YB wishes to thank the Research School of Astronomy and Astrophysics at The Australian National University for hosting him on a sabbatical during 2016–2017. CF gratefully acknowledges funding provided by the Australian Research Council’s Discovery Projects (grants DP150104329 and DP170100603). MRK’s work was supported under the Australian Research Council’s Discovery Projects funding scheme (project DP160100695). The simulations presented in this work used high performance computing

Publisher Copyright:
© 2017 The Author(s).


  • Dynamo
  • ISM: clouds
  • ISM: magnetic fields
  • Magnetohydrodynamics (MHD)
  • Plasmas
  • Turbulence


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