4.7 Article

Magnetic fields in late-stage proto-neutron stars

Journal

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 503, Issue 1, Pages 875-895

Publisher

OXFORD UNIV PRESS
DOI: 10.1093/mnras/stab460

Keywords

stars: evolution; stars: interiors; stars: neutron; stars: magnetic fields; stars: rotation

Funding

  1. European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant via National Science Centre (NCN), Poland [665778, UMO-2016/21/P/ST9/03689]
  2. NCN [2018/29/B/ST9/02013MF, 2017/26/D/ST9/00591]
  3. PHAROS COST Action [CA16214]

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The study explores the thermal and magnetic field structure of a late-stage proto-neutron star, establishing a simplified equation of state for hot neutron stars and solving the stellar equilibrium equations numerically. The ellipticity increases with temperature for a fixed magnetic field strength, and the Keplerian velocity is considerably lower for hotter stars. Magnetic fields stronger than around 10^14 G have qualitatively similar equilibrium states in both hot and cold neutron stars, with the poloidal field component dominating over the toroidal one.
We explore the thermal and magnetic field structure of a late-stage proto-neutron star (proto-NS). We find the dominant contribution to the entropy in different regions of the star, from which we build a simplified equation of state (EOS) for the hot neutron star (NS). With this, we numerically solve the stellar equilibrium equations to find a range of models, including magnetic fields and rotation up to Keplerian velocity. We approximate the EOS as a barotrope, and discuss the validity of this assumption. For fixed magnetic field strength, the induced ellipticity increases with temperature; we give quantitative formulae for this. The Keplerian velocity is considerably lower for hotter stars, which may set a de facto maximum rotation rate for non-recycled NSs well below 1 kHz. Magnetic fields stronger than around 10(14) G have qualitatively similar equilibrium states in both hot and cold NSs, with large-scale simple structure and the poloidal field component dominating over the toroidal one; we argue this result may be universal. We show that truncating magnetic field solutions at low multipoles leads to serious inaccuracies, especially for models with rapid rotation or a strong toroidal-field component.

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