Magnetic pressure anisotropy in ultra-dense white dwarfs: Landau quantization and neutronization effects

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_∥ P r = P ‖ and Pₜ = P_⊥ 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 12 C , ¹⁶O 16 O , ²⁰Ne 20 Ne , and ²⁴Mg 24 Mg compositions, as well as for fixed 50/50 C/O, Ne/O, and Mg/Ne mixtures. The EoS predicts P_⊥ < P_∥ 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) M ( ρ c ) curves reveal portions with dM/dρ c<0 d M / d ρ c < 0 that are flagged by the turning-point diagnostic after the maximum mass is reached. Our results show that strong magnetic fields significantly reduce the radii of low-mass WDs, placing the calculated branches in the same broad mass–radius domain as a selected observational sample, without constituting a specific observational identification or statistical validation, 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.