Research

Dynamo theory

Dynamo action is a mechanism enabling a conducting fluid that harbours three-dimensional and complex enough motions to amplify and self-sustain its own magnetic field. The latter generally have feedback reaction on the fluid motions, which makes the study of dynamo very challenging. The diversity of processes causing these complex motions differentiate the variety of dynamo mechanisms, which produce the majority of magnetic fields observed in the Universe. My analytical and numerical work focuses one particular dynamo, called the Tayler-Spruit dynamo, which is driven by the combination of large-scale differential rotation and the Tayler instability (instability of a strong large-scale toroidal magnetic field). This mechanism is suspected to produce very efficient angular momentum transport and may produce the extreme magnetic fields of magnetars in proto-neutron stars spun up by supernova fallback. Here are some good reviews:

  • Rincon 2019
  • Tobias 2021

    • Magnetic field stable configuration

    The magnetic fields observed at the surface of massive stars (spectal types A, B, and O) harbour a large-scale geometry (e.g. mostly dipolar configuration) and can reach high intensities (up to a few tens kiloGauss). Recent asteroseismic observations in red giants (i.e. evolved low-mass stars) also find very strong radial magnetic fields reaching a few hundreds of kiloGauss. While operating dynamo processes could be a natural explanation of these fields, it is difficult for them to form such strong magnetic fields, especially regarding the slow rotation of these stars. An alternative suggestion is that the fields stem from a past dynamo that operated during the star formation or stellar mergers. Once the dynamo stops operating, the remaining magnetic fields see their geometry reconfigurated to reach a stable configuration (i.e. not prone to any MHD instabilities) and finally dissipate at resisitive timescales. This is the so-called fossil field hypothesis and is promising to explain the observations. The research therefore focuses on finding these stable configurations to compare them to observations. In the context of magnetar formation, this problem is crucial to test the fossil field and dynamo scenarios as it would bridge the gap between the observation of magnetars, which are already evolved neutron stars, and the proto-neutron star MHD simulations. Indeed, understanding these configurations would allow to determine the observable signatures of the different scenarios. I participated, in a collaboration with my PhD team (R. Raynaud, J. Guilet) and a team from Leeds University (A. Igoshev, T. Woods, R. Hollerbach), in the first study investigating this question (accepted in Nature Astronomy).

    Magnetar formation

    Magnetars are isolated young neutron stars characterised by the most intense magnetic fields known in the Universe, which power a wide variety of high-energy emissions from giant flares to fast radio bursts. The origin of their magnetic field is still a challenging question. As supported by 3D numerical simulations, in situ magnetic field amplification by dynamo action (driven by convection, magnetorotational instability) could potentially generate ultra-strong magnetic fields in fast-rotating progenitors. However, it is unclear whether the fraction of progenitors harbouring fast core rotation is sufficient to explain the entire magnetar population. To address this point, we proposed a new scenario for magnetar formation involving a slowly rotating progenitor, in which a slow-rotating proto-neutron star is spun up by the supernova fallback. We argue that this can trigger the development of the Tayler-Spruit dynamo while other dynamo processes are disfavoured.

    Magnetic fields in stars

    From the observation of solar cycles to the measurements of the magnetic fields at the surface of some stars via the Zeeman effect, it is known that stars harbour magnetic fields from less than a few Gauss to a few hundred kiloGauss. Their presence in the stellar interior can influence the stellar evolution by influencing the transport of angular momentum and the chemical mixing. Recent asteroseismic measurements of (near-)core and surface rotation of some stars have shown that the stellar cores rotate much slower than expected by "non-magnetic" stellar evolution models. Magnetic fields are promising candidates (alongside gravity waves) to explain such discrepancies. It is therefore crucial to constrain the physics of magnetic fields in stellar interiors to reproduce the observations and better understand stellar evolution. The Tayler-Spruit dynamo is a promising mechanism to explain the observed efficient transport of angular momentum as it could produce strong enough large-scale magnetic fields. Thus, I am currently investigating further the physics of this dynamo process in order to provide better physical constrains for stellar evolution models.