A large-scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae

Abstract

Magnetohydrodynamic turbulence is important in many high-energy astrophysical systems, where instabilities can amplify the local magnetic field over very short timescales. Specifically, the magnetorotational instability and dynamo action have been suggested as a mechanism for the growth of magnetar-strength magnetic fields (of 10(15) gauss and above) and for powering the explosion of a rotating massive star. Such stars are candidate progenitors of type Ic-bl hypernovae, which make up all supernovae that are connected to long γ-ray bursts. The magnetorotational instability has been studied with local high-resolution shearing-box simulations in three dimensions, and with global two-dimensional simulations, but it is not known whether turbulence driven by this instability can result in the creation of a large-scale, ordered and dynamically relevant field. Here we report results from global, three-dimensional, general-relativistic magnetohydrodynamic turbulence simulations. We show that hydromagnetic 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 γ-ray bursts, and provide a viable mechanism for the formation of magnetars. Moreover, our findings suggest that rapidly rotating massive stars might lie behind potentially magnetar-powered superluminous supernovae.

The two promising scenarios to explode core collapse supernovae

“…Such simulations are highly resourcedemanding, and are on the limit of the possibility on the best computers. Mösta et al (2015), for example, performed simulations of CCSNe with pre-collapse rapidly rotating cores at very high resolutions. They found rapidly rotating material around the newly born neutron star, and that this material amplifies magnetic fields.…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…A logical indication is that they originate from massive stars which are fast rotators with initially strong magnetic fields. Such objects, with assumed initial rotation rate and magnetic fields, have been modeled [45,50,80], called here magneto-rotational or MHD-jet supernovae. The result is typically (when starting with initial fields of the order 10 12 Gauss) that the winding up of magnetic fields results in strong magnetic pressure along the polar rotation axis and jet-like ejection of matter.…”

“…A fully selfconsistent treatment would require high resolution simulations which can resolve magneto-rotational instabilities (MRI) and would predict reliably the possible amplification of magnetic fields during the explosion. Present calculations depend on the assumed initial conditions, which either cause strong jet ejection or can develop kink instabilities of the jets [45]. Fig.1b shows the nucleosynthesis results of the 3D collapse of a fast rotator with a strong initial magnetic field of 5x10 12 Gauss.…”

“…Therefore the dominant Y e in matter has to be of the order of 0.5. High explosion energies lead to high entropies and a strong alpha-rich freeze-out, including interesting amounts of 45 Sc, 64 Zn and other Fe-group elements. Magnetars have also been suggested to explain lGRBs (see previous subsection).…”

“…Finally, polar jets from fast rotators with strong magnetic fields, leading to a rare class of CCSNe producing magnetars. These objects have been discussed in detail in section 2.3 [45,50,80], they are rare (0.1% to 1% of regular CCSNe), do not produce much Ni/Fe, but large/comparable amounts of heavy relements as neutron star mergers. As a result of this discussion only 3/4 and 6 remain as possible sites to produce heavy r-elements and we will discuss them in the following with respect to their appearance in the chemical evolution of galaxies.…”