{
  "id": "2111.02148",
  "version": "v1",
  "published": "2021-11-03T11:36:52.000Z",
  "updated": "2021-11-03T11:36:52.000Z",
  "title": "MRI-driven $α-Ω$ dynamos in protoneutron stars",
  "authors": [
    "Alexis Reboul-Salze",
    "Jérôme Guilet",
    "Raphaël Raynaud",
    "Matteo Bugli"
  ],
  "comment": "Submitted to A&A, 20 pages, 19 figures",
  "categories": [
    "astro-ph.HE",
    "astro-ph.SR",
    "physics.flu-dyn"
  ],
  "abstract": "Magnetars are highly magnetized neutron stars that can produce X-ray and soft gamma-ray emissions and that have a dipole of $10^{14}$ G to $10^{15}$ G. A promising mechanism to explain magnetar formation is magnetic field amplification by the MRI in fast-rotating protoneutron stars (PNS). This scenario is supported by recent global models showing that small-scale turbulence can generate a dipole with magnetar-like intensity. However, the impact of buoyancy and density stratification on the efficiency of the MRI at generating a dipole is still unknown. We assess the impact of the density and entropy profiles on the MRI dynamo in a global model of a fast-rotating PNS, which focuses on its outer stratified region stable to convection. Using the pseudo-spectral code MagIC, we perform three-dimensional Boussinesq and anelastic MHD simulations in spherical geometry with explicit diffusivities. We perform a parameter study in which we investigate the effect of different approximations and of thermal diffusion. We obtain a self-sustained turbulent MRI-driven dynamo, which confirms most of our previous incompressible results once rescaled for density. The MRI also generates a non-dominant equatorial dipole, which represents about 4.3% of the averaged magnetic field strength. Interestingly, in the presence of a density gradient, an axisymmetric magnetic field at large scales oscillates with time, which can be described as a mean-field $\\alpha-\\Omega$ dynamo. Buoyancy damps turbulence in the equatorial plane but it has overall a relatively weak influence with a realistic high thermal diffusion. Our results support the ability of the MRI to generate magnetar-like large-scale magnetic fields. They furthermore predict the presence of an $\\alpha-\\Omega$ dynamo in the protoneutron star, which could be important to model in-situ magnetic field amplification in core-collapse supernovae. [abridged]",
  "revisions": [
    {
      "version": "v1",
      "updated": "2021-11-03T11:36:52.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "protoneutron star",
      "mri-driven",
      "model in-situ magnetic field amplification",
      "realistic high thermal diffusion",
      "generate magnetar-like large-scale magnetic fields"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 20,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}