We investigate the driving of MHD turbulence by gravitational contraction using simulations of an initially spherical, isothermal, magnetically supercritical molecular cloud core with transonic and trans-Alfvénic turbulence. We perform a Helmholtz decomposition of the velocity field, and investigate the evolution of its solenoidal and compressible parts, as well as of the velocity component along the gravitational acceleration vector, a proxy for the infall component of the velocity field. We find that (1) In spite of being supercritical, the core first contracts to a sheet perpendicular to the mean magnetic field, and the sheet itself collapses. (2) The solenoidal component of the turbulence remains at roughly its initial level throughout the simulation, while the compressible component increases continuously, implying that turbulence does not dissipate towards the centre of the core. (3) The distribution of simulation cells in the B–ρ plane occupies a wide triangular region at low densities, bounded below by the expected trend for fast MHD waves (B ∝ ρ, applicable for high-local Alfvénic Mach number MA) and above by the trend expected for slow waves (B ∼ constant, applicable for low local MA). At high densities, the distribution follows a single trend $B \propto \rho ^{\gamma _{\rm eff}}$, with 1/2 < γeff < 2/3, as expected for gravitational compression. (4) The mass-to-magnetic flux ratio λ increases with radius r due to the different scalings of the mass and magnetic flux with r. At a fixed radius, λ increases with time due to the accretion of material along field lines. (5) The solenoidal energy fraction is much smaller than the total turbulent component, indicating that the collapse drives the turbulence mainly compressibly, even in directions orthogonal to that of the collapse.
Many recent works on the observed galaxy clusters in the X-rays highlight broadly two classes of exclusive energy carriers – sound waves and turbulence. In order to understand this dichotomy, we design an idealized three-dimensional hydrodynamic simulation of a cluster, to assess which of these carriers can dissipate energy in and around the core (≳100 kpc). Specifically, we explore how gentle (long-duration outbursts) and intermediate (shorter duration outbursts) feedback modes can function efficiently mediated by compressible (sound waves) and incompressible (g modes/instabilities/turbulence) disturbances. Since g modes are confined tightly to the central core, we attempt to maximize the flux of fast sound waves to distribute the feedback energy over a large distance. We find that the contribution to heat dissipation from sound and turbulence varies on the basis of the aforementioned feedback modes, namely: turbulence contributes relatively more than sound in the slow-piston regime and vice versa for the intermediate regime. For the first time in a 3D simulation, we show that up to $\lesssim 20{{\ \rm per\ cent}}$ (in some directions) of the injected power can be carried away by sound flux in the intermediate feedback but it reduces to $\lesssim 10 {{\ \rm per\ cent}}$ (in some directions) in the slow-piston regime. Lastly, we find that sound waves can be elusive if we deduce the equation of state (isobaric/isentropic) of the fluctuations from X-ray observations.
more » « less- PAR ID:
- 10368365
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 514
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3765-3788
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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