Magnetic field amplification and magnetically supported explosions of collapsing, non-rotating stellar cores

Abstract

We study the amplification of magnetic fields in the collapse and the post-bounce evolution of the core of a non-rotating star of 15 M in axisymmetry. To this end, we solve the coupled equations of magnetohydrodynamics and neutrino transport in the two-moment approximation. The pre-collapse magnetic field is strongly amplified by compression in the infall. Initial fields of the order of 10 10 G translate into proto-neutron star fields similar to the ones observed in pulsars, while stronger initial fields yield magnetar-like final field strengths. After core bounce, the field is advected through the hydrodynamically unstable neutrino-heating layer, where non-radial flows due to convection and the standing accretion shock instability amplify the field further. Consequently, the resulting amplification factor of order five is the result of the number of small-eddy turnovers taking place within the time scale of advection through the post-shock layer. Due to this limit, most of our models do not reach equipartition between kinetic and magnetic energy and, consequently, evolve similarly to the non-magnetic case, exploding after about 800 ms when a single or few highentropy bubbles persist over several dynamical time scales. In the model with the strongest initial field we studied, 10 12 G, for which equipartition between flow and field is achieved, the magnetic tension favours a much earlier development of such long-lived high-entropy bubbles and enforces a fairly ordered large-scale flow pattern. Consequently, this model, after exhibiting very regular shock oscillations, explodes much earlier than non-magnetic ones.

Evolution of the surface magnetic field of rotating proto-neutron stars

“…These processes are complemented by the possible amplification of magnetic fields in the interior of the PNS, in particular if rotation and convection constitute a dynamo. Previously, we studied the evolution of magnetized core collapse for stars with an initial mass of 15 M without rotation [11]. Varying the pre-collapse field strength between negligible values below 10 10 G and dynamically relevant, yet from the point of view of stellar evolution rather overestimated, values of 10 12 G, we found explosions driven by neutrino heating with potentially a strong contribution of magnetic forces.…”

Discovery of a Strongly Phase-Variable Spectral Feature in the Isolated Neutron Star Rx J0720.4–3125

“…If the feature arises from bound-bound (or bound-free) transitions, the atmosphere composition is most probably He or mid-Z elements (see, e.g., Pavlov & Bezchastnov 2005;Mori & HO 2007 The presence of proton cyclotron lines produced by resonant scattering/absorption in confined, high-B structures close to the star surface, where the magnetic field is a factor 10 higher than the dipole, would be supportive of a picture in which the magnetic field of (highly magnetized) neutron stars is complex, with substantial deviations from a pure dipole on the small scales. In recent years, a better (although far from conclusive) understanding of how stellar magnetic fields are generated and amplified indicates that strong, non-dipolar field components are ubiquitous, from massive stars (Braithwaite & Spruit 2006) to proto-neutron stars (Obergaulinger et al 2014). It is now well established that internal toroidal field components, multipolar surface structures, as well as very localized B-field bundles must be present in many neutron stars, especially in highly magnetized ones.…”

Protomagnetar and black hole formation in high-mass stars

“…To this end, we use the Eulerian finite-volume code described in Just et al (2015), which we previously had applied in core-collapse modelling (Obergaulinger et al 2014). The most important extension w.r.t.…”