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キーワード:
Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
要旨:
Magnetohydrodynamic (MHD) turbulence is of key importance in many high-energy
astrophysical systems, including black-hole accretion disks, protoplanetary
disks, neutron stars, and stellar interiors. MHD instabilities can amplify
local magnetic field strength over very short time scales, but it is an open
question whether this can result in the creation of a large scale ordered and
dynamically relevant field. Specifically, the magnetorotational instability
(MRI) has been suggested as a mechanism to grow magnetar-strength magnetic
field ($\gtrsim 10^{15}\, \mathrm{G}$) and magnetorotationally power the
explosion of a rotating massive star. Such stars are progenitor candidates for
type Ic-bl hypernova explosions that involve relativistic outflows and make up
all supernovae connected to long gamma-ray bursts (GRBs). We have carried out
global 3D general-relativistic magnetohydrodynamic (GRMHD) turbulence
simulations that resolve the fastest growing mode (FGM) of the MRI. We show
that MRI-driven MHD turbulence in rapidly rotating protoneutron stars produces
a highly efficient inverse cascade of magnetic energy. This builds up magnetic
energy on large scales whose magnitude rivals the turbulent kinetic energy. We
find a large-scale ordered toroidal field along the rotation axis of the
protoneutron star that is consistent with the formation of bipolar
magnetorotationally driven outflows. Our results demonstrate that rapidly
rotating massive stars are plausible progenitors for both type Ic-bl supernovae
and long GRBs, present a viable formation scenario for magnetars, and may
account for potentially magnetar-powered superluminous supernovae.
