2024-04-07 06:00:12
Magnetars are neutron stars with the most intense magnetic fields observed in the Universe.
To tackle the still open question of the origin of these extreme magnetic fields, a scenario was proposed by a team from the Department of Astrophysics (DAp) of CEA Saclay using the Tayler-Spruit dynamo mechanism, caused by the material that falls on the young neutron star after theexplosion en supernovae The team of scientists showed in 2022 through an analytical analysis that this type of dynamo could explain the intensity of the magnetic field of magnetars.
Credit: ESO/L. Calçada
In this new study, the team confirms this result using three-dimensional numerical simulations. This will have great implications for understanding the origin of magnetic fields, not only for magnetars, but also for stellar evolution where the same dynamo mechanism could be at work.
This new study was published in the journal Monthly Notices of the Royal Astronomical Society: Letters.
Magnetar formation scenarios
Neutron stars are the result of the violent contraction of the iron core of a massive star during its gravitational supernova explosion. They have a radius of about 12 km and a mass of 1 to 2 times that of the Sun, which implies extreme density.
Magnetars are a special class of neutron stars. They emit primarily in X-rays due to the dissipation of an extremely intense magnetic field which, in combination with very rapid rotation, can cause extremely energetic supernovae, such as hypernovae and super-luminous supernovae.
The question of the formation of magnetars, as well as the associated extreme explosion scenario, is still hotly debated but a promising scenario is the amplification of the magnetic field by a dynamo effect in a proto-star neutron.
Dynamo effects, which are certainly at the origin of the majority of astrophysical magnetic fields, are complex instability processes coupling the movements of a fluid and its magnetic field to amplify and maintain the latter in a self-sustaining manner.
FIGURE 2 – Three-dimensional visualization of the magnetic field lines and meridional section of the cylindrical radial component of the magnetic field of the hemispherical (left) and dipolar (right) Tayler-Spruit dynamos.
Two types of dynamos were studied within the supernovae team of the Department of Astrophysics at CEA Saclay, one is maintained by convective movements and the other by an instability magnetohydrodynamic due to the differential rotation fluid. These scenarios make it possible to find the intensity of the magnetic field of magnetars and the extreme explosions.
However, these dynamos require the core of the progenitor star to be rotating rapidly, which is still uncertain and probably too rare to explain the entire observed population.
A third scenario proposed by this same team suggests that the rapid rotation of the proto-neutron star comes from the material falling on the surface of the latter approximately 10 seconds after the start of the explosion. This triggers a dynamo maintained by the Tayler-Spruit instability, an instability of the toroidal magnetic field when it becomes too intense, at the expense of the two other dynamo mechanisms.
“This new scenario complements the previous two, because it does not require a rapidly rotating core to generate a magnetar. Thus, it applies more to magnetars formed within supernovae with typical energies, while the other two are more suitable for magnetars born in more energetic explosions, therefore implying a rapidly rotating core.” specifies Paul Barrère, searcher main point of this study.
Figure 3 – Intensity of the magnetic dipole as a function of the variation of the rotation speed along the radius measured locally in the simulations of dipolar (in red) and hemispherical (in green) dynamos. These simulations are in agreement with the theoretical predictions (dashed lines) of Fuller et al. 2019 (in red) and Spruit 2002 (in green).
The contribution of three-dimensional numerical simulations in the confirmation of results
The existence of the Tayler-Spruit dynamo has long been debated. After one first analytical study showing that this mechanism can participate in the formation of magnetarsresearchers from the supernovae team of the DAp and the Max Planck Institute for Gravitational Physics have taken a decisive step forward by studying it through three-dimensional numerical simulations. These simulations resulted in the reproduction of the magnetic dipole, with an intensity of 1014 G, corresponding to the order of magnitude observed in magnetars, thus confirming the existence of the Tayler-Spruit dynamo. Added to this leading result is the discovery that there are two types of Tayler-Spruit dynamo which are distinguished by the intensity and geometry of the magnetic field that they generate. In Figure 2, we can see that the magnetic field is either concentrated in one hemisphere (left) or has dipolar symmetry with respect to the equator (right). Furthermore, these Tayler-Spuit dynamos are respectively in agreement with the theoretical predictions of Spruit 2002 and Fuller et al. 2019 (see Figure 3), accentuating the relevance of this new scenario.
“In addition to taking a big step in understanding the formation of magnetars, this study will have a broader impact in the field of stellar physics, particularly on the problem of transport the moment kinetic at the origin of slow-down of the rotation of the hearts of stars.” said Paul Barrère.
The team is currently continuing this work by studying the impact of different physical parameters on Tayler-Spruit dynamos, with the aim of better understanding the formation of magnetars and deepening our understanding of these dynamos.
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