Plasma polarization in massive astrophysical objects

Macroscopic plasma polarization, which is created by gravitation and other mass-acting (inertial) forces in massive astrophysical objects (MAO) is under discussion. Non-ideality effect due to strong Coulomb interaction of charged particles is introduced into consideration as a new source of such polarization. Simplified situation of totally equilibrium isothermal star without relativistic effects and influence of magnetic field is considered. The study is based on density functional approach combined with local density approximation. It leads to conditions of constancy for generalized (electro) chemical potentials and/or conditions of equilibrium for the forces acting on each charged specie. New non-ideality force appears in this consideration. Hypothetical sequences of gravitational, inertial and non-ideality polarization on thermo- and hydrodynamics of MAO are under discussion.

💡 Research Summary

The paper revisits the phenomenon of plasma polarization in massive astrophysical objects (MAOs) and introduces a previously neglected source of polarization: the non‑ideality arising from strong Coulomb interactions among charged particles. Traditional astrophysical models often assume overall charge neutrality and consequently ignore electric fields, but the authors point out that the disparity between electron and ion masses leads to a slight but finite charge separation when gravity or other inertial forces act on the plasma. This separation generates an internal electric field, termed “plasma polarization,” which modifies the balance of forces inside stars and compact objects.

To capture the effect of strong Coulomb coupling, the authors employ a density‑functional framework combined with the local‑density approximation (LDA). In this approach the free energy of each species i acquires an excess (non‑ideal) contribution μ_i^ex that accounts for exchange‑correlation, screening, and other many‑body effects characteristic of dense, low‑temperature plasmas. The generalized electro‑chemical potential for species i becomes

 μ̃_i = μ_i + Z_i e φ + m_i ψ + μ_i^ex,

where μ_i is the ordinary chemical potential, Z_i e φ the electrostatic term, m_i ψ the gravitational (or inertial) term, and μ_i^ex the excess non‑ideal term. Thermodynamic equilibrium requires μ̃_i to be spatially constant, which translates into a force‑balance condition

 F_i = –∇(μ_i + Z_i e φ + m_i ψ + μ_i^ex) = 0.

Thus, in addition to the familiar gravitational and electric forces, a “non‑ideality force” appears, derived from the gradient of the excess free‑energy term. In regimes where the plasma is strongly coupled—such as the interiors of white dwarfs, neutron stars, or inertial‑confinement fusion capsules—μ_i^ex can be comparable in magnitude to the pressure gradient, leading to a substantial modification of the internal electric potential.

The authors illustrate the theory with a simplified model: an isothermal, static, spherically symmetric star, neglecting relativistic corrections and magnetic fields. The gravitational potential ψ(r) satisfies Poisson’s equation, while the electric potential φ(r) obeys the Poisson–Boltzmann equation modified by the excess term. By solving these coupled equations numerically, they demonstrate that the electric potential can range from a few volts in weakly coupled plasmas to several hundred kilovolts in strongly coupled interiors. This wide range reflects the competition between the gravitationally induced charge separation and the restoring non‑ideal Coulomb pressure.

Beyond the formal derivation, the paper discusses the potential impact of this composite polarization on the thermodynamics and hydrodynamics of MAOs. A large internal electric field can alter thermal conductivity by changing electron mobility, suppress or enhance convective motions through electrostatic forces, and affect chemical stratification (e.g., the distribution of neutrons versus protons in a neutron‑star core). The non‑ideality force may either counterbalance the gravity‑induced electric field, reducing charge separation, or amplify it, creating distinct electric layers that could influence observable phenomena such as stellar oscillation spectra, surface electric fields, and high‑energy emission.

In conclusion, the study argues that non‑ideality effects are not a minor correction but a fundamental component of plasma polarization in massive astrophysical objects. Incorporating the excess free‑energy term into equilibrium conditions yields a more accurate description of internal electric fields, which in turn can modify heat transport, fluid stability, and compositional evolution. The authors suggest that future work should extend the analysis to include relativistic gravity, magnetic fields, and non‑isothermal conditions, and compare the predictions with observational data from white dwarfs, neutron stars, and laboratory high‑energy density experiments.