To explain the genesis of magnetars’ magnetic field, we developed new and unprecedently detailed numerical models describing the amplification of pre-existing weak fields when neutron stars are born with fast rotation. The two most likely mechanisms consist either in a convective dynamo similar to planetary and stellar dynamos or in tapping into the differential rotation energy thanks to the magnetorotational instability (MRI). Although they are well studied in other astrophysical contexts, the effectiveness of these two physical mechanisms under the physical conditions specific to protoneutron stars was poorly understood. Their capacity to generate a magnetic field coherent over the whole neutron star was particularly uncertain, although this is crucial to explain the properties of galactic magnetars and to power an explosion. We produced the first 3D numerical simulations of each of these two processes in a protoneutron star model describing its full angular extent. To develop these models, we have adapted numerical methods from the field of stellar and planetary dynamos to the context of a proto-neutron star with ultra-dense and hot matter. These simulations showed for the first time the generation of a magnetic field consistent with the observational constraints from galactic magnetars. They showed in particular the appearance for fast enough rotation rates of a new branch of convective dynamo with much larger magnetic fields. Not only does the dipolar magnetic field reach 10^15 G, but the toroidal component is even stronger by a factor ten. These simulations also unraveled an enexpected property of the MRI dynamo: the magnetic dipole is tilted preferentially toward the equator.
During the course of the project, we also proposed a novel scenario for magnetar formation, which is motivated by recent simulations of supernovae explosions suggesting the importance of matter falling back onto the nascent neutron star. Contrary to previous scenarios assuming a fast-rotating progenitor, this novel scenario starts from a slowly rotating progenitor and assumes that the proto-neutron star is spun up by fallback matter. We argued that this can trigger the development of the so-called Tayler-Spruit dynamo while other dynamo processes previously considered are disfavored in this case. Using both semi-analytical modeling and 3D numerical simulations we demonstrated that magnetars can be formed in this scenario with plausible assumptions on the fallback mass. These results are not only important to understand magnetar formation but also for the fields of dynamo theory and stellar evolution as the Tayler-Spruit dynamo had until recently remained elusive in numerical simulations.
In addition to shedding light on Galactic magnetar formation, these results open new avenues to understand the most powerful and most luminous explosions of massive stars. Until now, the main weakness of the millisecond magnetar scenario was to assume an ad hoc strong magnetic field, independent of the fast rotation rate of the neutron star. Our results provide the first numerical demonstration that the right magnetic field can be generated consistently with the fast rotation needed to power the explosion. Another important result has been that the topology of the magnetic field is much more complex than the standard purely dipolar field assumed by most numerical models of magnetorotational explosions. We therefore studied the impact of a more complex geometry of the magnetic field by carrying out numerical simulations of such explosions with different magnetic field configurations. We have shown that the geometry of the magnetic field plays an important role to determine the properties of the explosion. Magnetic fields with more large-scale coherence lead to more energetic explosions that are more collimated in the form of jets. We also showed that the properties of the magnetic field play a major role in determining the gravitational wave signal and the heavy element nucleosynthesis yields. These results demonstrate the need to better describe the origin and properties of the magnetic field to achieve a realistic description of extreme explosions.
