Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects
Abstract
Strong magnetic fields modify the thermodynamics of white dwarf (WD) matter by quantizing the transverse motion of degenerate electrons into Landau levels and splitting the pressure into components parallel and perpendicular to the magnetic field. We investigate how this microscopic pressure anisotropy affects ultra-dense WD sequences when Coulomb lattice contributions, electron capture thresholds, neutron drip limits, and nuclear composition are treated consistently in the equation of state (EoS). The stellar equilibrium structure is obtained by solving an anisotropic extension of the Tolman--Oppenheimer--Volkoff (TOV) equations, where Pr = P∥ and Pt = P⊥ denote the radial and tangential pressures, respectively. We compare the post-electron-capture branches with their corresponding pre-capture sequences for pure 12C, 16O, 20Ne, and 24Mg compositions, as well as for fixed 50/50 C/O, Ne/O, and Mg/Ne mixtures. The EoS predicts P⊥ < P∥ in density ranges where only a few Landau levels are occupied. Consequently, pressure anisotropy shifts the stellar sequences toward smaller radii, with only minor changes in the maximum masses. We also find that post-capture configurations cannot be interpreted from the mass--radius plane alone, since the corresponding M(ρc) curves reveal unstable portions after the maximum mass is reached. Our results show that strong magnetic fields significantly reduce the radii of low-mass WDs, yielding better agreement with the observational data available in the Montreal White Dwarf Database and suggesting that future high-precision observations of massive magnetic WDs could help assess the role of magnetic pressure anisotropy in determining their masses and radii.
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BibTeX
@misc{otoniel2026magnetic,
title = {{Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects}},
author = {Edson Otoniel and Juan M. Z. Pretel and Victor B. T. Alves and César O. V. Flores and Clésio E. Mota},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2607.25104},
eprint = {2607.25104},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.25104},
}
RIS
TY - UNPB
TI - Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects
AU - Edson Otoniel
AU - Juan M. Z. Pretel
AU - Victor B. T. Alves
AU - César O. V. Flores
AU - Clésio E. Mota
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2607.25104
UR - https://arxiv.org/abs/2607.25104
AB - Strong magnetic fields modify the thermodynamics of white dwarf (WD) matter by quantizing the transverse motion of degenerate electrons into Landau levels and splitting the pressure into components parallel and perpendicular to the magnetic field. We investigate how this microscopic pressure anisotropy affects ultra-dense WD sequences when Coulomb lattice contributions, electron capture thresholds, neutron drip limits, and nuclear composition are treated consistently in the equation of state (EoS). The stellar equilibrium structure is obtained by solving an anisotropic extension of the Tolman--Oppenheimer--Volkoff (TOV) equations, where P_r = P_∥ and P_t = P_⊥ denote the radial and tangential pressures, respectively. We compare the post-electron-capture branches with their corresponding pre-capture sequences for pure ^12C, ^16O, ^20Ne, and ^24Mg compositions, as well as for fixed 50/50 C/O, Ne/O, and Mg/Ne mixtures. The EoS predicts P_⊥ < P_∥ in density ranges where only a few Landau levels are occupied. Consequently, pressure anisotropy shifts the stellar sequences toward smaller radii, with only minor changes in the maximum masses. We also find that post-capture configurations cannot be interpreted from the mass--radius plane alone, since the corresponding M(ρ_c) curves reveal unstable portions after the maximum mass is reached. Our results show that strong magnetic fields significantly reduce the radii of low-mass WDs, yielding better agreement with the observational data available in the Montreal White Dwarf Database and suggesting that future high-precision observations of massive magnetic WDs could help assess the role of magnetic pressure anisotropy in determining their masses and radii.
ER -