A large scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae (1512.00838v2)

Abstract: Magnetohydrodynamic (MHD) turbulence is of key importance in many high-energy astrophysical systems, where MHD instabilities can amplify local magnetic field over very short time scales. Specifically, the magnetorotational instability (MRI) and dynamo action have been suggested as a mechanism to grow magnetar-strength magnetic field ($\ge10^{15} G$) and magnetorotationally power the explosion of a rotating massive star. Such stars are progenitor candidates for type Ic-bl hypernova explosions and make up all supernovae connected to long gamma-ray bursts (GRBs). The MRI has been studied with local high-resolution shearing box simulations in 3D or with global 2D simulations, but it is an open question whether MRI-driven turbulence can result in the creation of a large-scale ordered and dynamically relevant field. Here we report results from global 3D general-relativistic magnetohydrodynamic (GRMHD) turbulence simulations and show that MRI-driven MHD turbulence in rapidly rotating protoneutron stars produces an inverse cascade of energy. We find a large-scale ordered toroidal field 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.

• The paper investigates magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI) using high-resolution 3D general-relativistic simulations of rapidly rotating core-collapse supernovae to explore the generation of large-scale magnetic fields.
• Simulations show a strong resolution dependency, with higher resolutions (100m, 50m) exhibiting exponential magnetic field growth, rapid saturation, and an inverse cascade of magnetic energy consistent with MRI-driven turbulence.
• These findings support MRI-driven turbulence as a plausible mechanism for generating magnetar-strength magnetic fields and explaining phenomena like gamma-ray bursts and specific explosion geometries in rapidly rotating core-collapse supernovae.

Investigating Magnetoturbulence and Dynamo Action in Rapidly Rotating Core-Collapse Supernovae

The paper conducted by Mösta et al. explores the intricacies of magnetohydrodynamic (MHD) turbulence within rapidly rotating core-collapse supernovae, focusing on magnetorotational instability (MRI) as a potential mechanism for magnetic field amplification. The researchers employ high-resolution global three-dimensional (3D) general-relativistic magnetohydrodynamic (GRMHD) simulations to explore whether MRI-driven turbulence can foster the development of a large-scale ordered magnetic field, relevant to powerful astrophysical phenomena such as magnetar-strength fields and magnetorotational explosions. These findings are crucial in understanding the formation and dynamics of progenitors of type Ic-bl supernovae, long gamma-ray bursts (GRBs), and potentially magnetar-powered superluminous supernovae.

Methodology and Resolution Dependency

Utilizing a higher resolution than prior studies, the global 3D GRMHD simulations paper the rotational shear layer around a rapidly spinning protoneutron star approximately 20 milliseconds after the core bounce. Initial conditions were derived from adaptive mesh refinement (AMR) simulations of a rotational stellar collapse. The paper deploys simulations across four varying spatial resolutions (500 m, 200 m, 100 m, 50 m), with a focus on capturing the fastest growing mode (FGM) of the MRI. The efficacy of MRI in turbulent field amplification is contingent upon resolving the FGM, achievable in the 100 m and 50 m resolution simulations.

The findings indicate that the higher resolution simulations prompt an exponential growth in the toroidal magnetic field, consistent with linear analysis predictions. The growth rate ($\tau \approx 0.5$ ms) aligns closely with analytical expectations. This phenomena transitions rapidly to a turbulent state and achieves a saturated field strength rapidly, within 3 ms.

Results and Implications

The outcome of the paper manifests a striking dependency on resolution, where lower resolution simulations (500 m and 200 m) show limited turbulence, in contrast to the 100 m and 50 m simulations which evidence a fully turbulent state. Specifically, the emergence of large-scale bipolar magnetorotational outflows aligns with the potential formation of a structured toroidal field. Furthermore, an inverse cascade of magnetic energy from small to large spatial scales predominantly characterizes the energy dynamics after the nonlinear transition. This suggests substantive energy redistribution, eventually leading to electromagnetic energy on par with turbulent kinetic energy. Noteworthy is the role of MHD stresses likely surpassing neutrino heating in reviving stalled shocks within rapidly rotating stars, indicating a major mechanism for producing magnetar-like magnetic fields during stellar collapses.

Conclusions and Future Directions

The paper supports the plausibility of rapidly rotating massive stars harboring MRI-driven turbulence as progenitors for magnetar-strength fields, explosive GRBs, and double-lobed explosion geometries characteristic of MHD-driven supernovae. Intensifying resolution in simulations might further elucidate details of magnetic saturation and energy cascade efficiency. The complexities observed in the dynamo action and chaotic turbulence layers underscore the necessity for integrating high-resolution MHD dynamics within broader-scale stellar collapse models, focusing on MHD-powered shock revival and explosion geometries.

The extension of these simulations with even greater resolution could refine understandings of secondary instabilities and offer deeper insights into magnetic field formation dynamics in protoneutron stars. This paper by Mösta et al. significantly advances the comprehension of magnetic field amplification in massive astrophysical collapse and offers a potentially integrated framework for the synthesis of observed high-energy astrophysical phenomena.