arXiv subjects

Peter B. Rau

Publications and source records attributed to Peter B. Rau.

11 recordsLinked to original sources

arXiv2026

Spin effects in superfluidity, neutron matter and neutron stars

We review selected aspects of the interior physics of compact stars, focusing on the microscopic and macroscopic manifestations of spin, magnetic fields, and nucleonic superfluidity and superconductivity. Spin statistics of fermions allows quantum degeneracy pressure to determine the stability and global properties of neutron stars, whose structure depends sensitively on the strong interactions among baryons in dense matter. Using a generic meta-modeling framework based on an expansion of the nuclear energy density around the isospin-symmetric and saturation-density limits, we highlight how various lesser-known terms in this expansion affect compact-star observables and review multimessenger constraints on mass, radius, and moment of inertia. The influence of magnetic fields on dense matter is examined, showing that substantial effects in their structure require extremely strong fields, whereas lower fields are sufficient to affect their superfluid phases. At the mesoscopic scale, the coexistence of superfluid and superconducting components features vortex and flux-tube lattices, with pinning and mutual friction processes playing central roles in neutron-star rotational dynamics. We discuss unresolved issues concerning vortex structure, flux-tube configurations, and the origin of pulsar glitches and post-glitch relaxation. We also briefly address the possible emergence of deconfined quark phases in compact-star cores, including their color-superconducting properties, as well as the associated vortex structures and magnetic-field responses in such phases.

astro-ph.HE↗

arXiv2025

Suppression of composition $g$-modes in chemically-equilibrating warm neutron stars

We investigate the impact of chemical equilibration and the resulting bulk viscosity on non-radial oscillation modes of warm neutron stars at temperatures up to $T\approx 5$ MeV, relevant for protoneutron stars and neutron-star post-merger remnants. In this regime, the relaxation rate of weak interactions becomes comparable to the characteristic frequencies of composition $g$-modes in the core, resulting in resonant damping. To capture this effect, we introduce the dynamical sound speed, a complex, frequency-dependent generalization of the adiabatic sound speed that encodes both the restoring force and the dissipative effects of bulk compression. Using realistic weak reaction rates and three representative equations of state, we compute the complex frequencies of composition $g$-modes with finite-temperature profiles. We find that bulk viscous damping becomes increasingly significant with temperature and can completely suppress composition $g$-modes. In contrast, the $f$-mode remains largely unaffected by bulk viscosity due to its nearly divergence-free character. Our results highlight the sensitivity of $g$-mode behavior to thermal structure, weak reaction rates, and the equation of state, and establish the dynamical sound speed as a valuable descriptor characterizing oscillation properties in dissipative neutron star matter.

astro-ph.HE↗

arXiv2025

Josephson currents in neutron stars

We demonstrate that the interface between $S$-wave and $P$-wave paired superfluids in neutron stars induces a neutron supercurrent, a charge-neutral analog of the Josephson junction effect in electronic superconductors. The proton supercurrent entrainment by the neutron superfluid generates, in addition to the neutral supercurrent, a charged current across the interface. Beyond this stationary effect, the motion of the neutron vortex lines responding to secular changes in the neutron star's rotation rate induces a time-dependent oscillating Josephson current across this interface when proton flux tubes are dragged along with them. We show that such motion produces radiation from the interface once clusters of proton flux tubes intersect the interface. The power of radiation exceeds by orders of magnitude the Ohmic dissipation of currents in neutron stars. This effect appears to be phenomenologically significant enough to heat the star and alter its cooling rate during the photon cooling era.

astro-ph.HE↗

arXiv2025

Numerical simulation of electron magnetohydrodynamics with Landau-quantized electrons in magnetar crusts

In magnetar crusts, magnetic fields are sufficiently strong to confine electrons into a small to moderate number of quantized Landau levels. This can have a dramatic effect on the crust's thermodynamic properties, generating field-dependent de Haas--van Alphen oscillations. We previously argued that the large-amplitude oscillations of the magnetic susceptibility could enhance the Ohmic dissipation of the magnetic field by continuously generating small-scale, rapidly dissipating field features. This could be important to magnetar field evolution and contribute to their observed higher temperatures. To study this, we performed quasi-3D numerical simulations of electron MHD in a representative volume of neutron star crust matter, for the first time including the magnetization and magnetic susceptibility resulting from Landau quantization. We find that the potential enhancement in the Ohmic dissipation rate due to this effect can be a factor $\sim 3$ for temperatures of the order of $10^8$ K and $\sim 4.5$ for temperatures of the order of $5\times10^7$ K, depending on the magnetic field configuration. The nonlinear Hall term is crucial to this amplification: without it the magnetic field decay is only enhanced by a factor $\lesssim 2$ even at $5\times10^7$ K. These effects generate a high wavenumber plateau in the magnetic energy spectrum associated with the small-scale de Haas--van Alphen oscillations. Our results suggest that this mechanism could help explain the magnetar heating problem, though due to the effect's temperature-dependence, full magneto-thermal evolution simulations in a realistic stellar model are needed to judge whether it is viable explanation.

astro-ph.HE↗

arXiv2023

Non-equilibrium effects on stability of hybrid stars with first-order phase transitions

The stability of hybrid stars with first-order phase transitions as determined by calculating fundamental radial oscillation modes is known to differ from the predictions of the widely-used Bardeen--Thorne--Meltzer criterion. We consider the effects of out-of-chemical-equilibrium physics on the radial modes and hence stability of these objects. For a barotropic equation of state, this is done by allowing the adiabatic sound speed to differ from the equilibrium sound speed. We show that doing so extends the stable branches of stellar models, allowing stars with rapid phase transitions to support stable higher-order stellar multiplets similarly to stars with multiple slow phase transitions. We also derive a new junction condition to impose on the oscillation modes at the phase transition. Termed the reactive condition, it is physically motivated, consistent with the generalized junction conditions between two phases, and has the common rapid and slow conditions as limiting cases. Unlike the two common cases, it can only be applied to nonbarotropic stars. We apply this junction condition to hybrid stellar models generated using a two-phase equation of state consisting of nuclear matter with unpaired quark matter at high densities joined by a first-order phase transition and show that like in the slow limiting case, stars that are classically unstable are stabilized by a finite chemical reaction speed.

astro-ph.HE↗

arXiv2023

Two first-order phase transitions in hybrid compact stars: higher-order multiplet stars, reaction modes and intermediate conversion speeds

We study compact stars with hybrid equations of state consisting of a nuclear outer region and two nested quark phases, each separated from the lower density phase by a strong first-order phase transition. The stability of these models is determined by calculating their radial oscillation modes with different conversion rates between adjacent phases and hence junction conditions for the modes at the phase separation interface between them. In the case when the timescale of transition is faster than the period of oscillations, we recover the traditional stability criterion implying that $\partial M/\partialρ_c>0$ on the stable branch(es), where $M$ is the mass and $ρ_c$ is the central density. In the opposite limit of slow conversion, we find stable stellar multiplets beyond triplets consisting of stars that are stable by the usual criterion plus slow-conversion (denoted by $s$) hybrid stars with $\partial M/\partialρ_c<0$ that are stabilized due to an alternative junction condition on the fluid displacement field at the interface reflecting the slow rate of conversion. We also study the properties of the reaction mode, the radial mode that only exists for stars with rapid (abbreviated by $r$) phase transitions, in stars with either two rapid phase transitions or alternating rapid and slow phase transitions for the first time. The implications of alternative junction conditions are also examined, with these conditions generally being found to provide stability properties similar to those for a slow conversion rate.

astro-ph.HE↗

arXiv2023

Magnetohydrodynamic stability of magnetars in the ultrastrong field regime II: The crust

We study the stability of Hall MHD with strong magnetic fields in which Landau quantization of electrons is important. We find that the strong-field Hall modes can be destabilized by the dependence of the differential magnetic susceptibility on magnetic field strength. This instability is studied using linear perturbation theory, and is found to have typical growth time of order $\lesssim 10^3$ yrs, with the growth time decreasing as a function of wavelength of the perturbation. The instability is self-limiting, turning off following a period of local field growth by a few percent of the initial value. Finite temperature is also shown to limit the instability, with sufficiently high temperatures eliminating it altogether. Alfvén waves can show similar unstable behaviour on shorter timescales. We find that Ohmic heating due to the large fields developed via the instability and magnetic domain formation is not large enough to account for observed magnetar surface temperatures. However, Ohmic heating is enhanced by the oscillatory differential magnetic susceptibility of Landau-quantized electrons, which could be important to magneto-thermal simulations of neutron star crusts.

astro-ph.HE↗

arXiv2021

Unstable modes of hypermassive compact stars driven by viscosity and gravitational radiation

We study the oscillations modes of differential rotating remnants of binary neutron star inspirals by modeling them as incompressible Riemann ellipsoids parametrized by the ratio $f$ of their internal circulation to the rotation frequency. The effects of viscosity and gravitational wave radiation on the modes are studied and it is shown that these bodies exhibit generic instabilities towards gravitational wave radiation akin to the Chandrasekhar--Friedman--Schutz instabilities for uniformly rotating stars. The odd-parity modes are unstable for all values of $f$ (except for the spherical model) and deformations, whereas the even parity unstable modes appear only in highly eccentric ellipsoids. We quantify the modification of the modes with varying mass of the model and the magnitude of the viscosity. The modes are weakly dependent on the range of the masses relevant to the binary neutron star mergers. Large turbulent viscosity can lead to a suppression of the gravitational wave instabilities, whereas kinematical viscosity has a negligible influence on the modes and their damping timescales.

gr-qc↗

arXiv2021

Magnetohydrodynamic stability of magnetars in the ultrastrong field regime I: The core

We study magnetohydrodynamic stability of neutron star core matter composed of neutrons, protons and leptons threaded by a magnetar-strength magnetic field $10^{14}$--$10^{17}$ G, where quantum electrodynamical effects and Landau quantization of fermions are important. Stability is determined using the Friedman--Schutz formalism for the canonical energy of fluid perturbations, which we calculate for a magnetizable fluid with $H\neq B$. Using this and the Euler--Heisenberg--Fermi--Dirac Lagrangian for a strongly magnetized fluid of Landau-quantized charged fermions, we calculate the local stability criteria for a fluid slab as a stand-in for a segment of a neutron star core, accounting for magnetic and composition gradient buoyancy. The slab is threaded by a field orthogonal to the gravitational field, the Cartesian analogy to a toroidal field. We find that, for sufficiently strong fields $B\gtrsim10^{15}$ G, the magnetized fluid is unstable to a magnetosonic-type instability with growth times of order $10^{-3}$ s. The instability is triggered by sharp changes in the second-order field derivative of the Euler--Heisenberg--Fermi--Dirac Lagrangian which occur where additional Landau levels start being populated. These sharp changes are divergent at zero temperature, but are finite for nonzero temperature, so realistic neutron star core temperatures $5\times10^7$ K$<T<5\times10^8$ K are used. We conjecture that this mechanism could promote the formation of magnetic domains as predicted by Blandford and Hernquist (1982) and Suh and Mathews (2010).

astro-ph.HE↗

arXiv2020

Oscillations of hypermassive compact stars with gravitational radiation and viscosity

Binary neutron star mergers, such as the multimessenger GW170817 event, may produce hypermassive compact objects which are supported against collapse by the internal circulation of the fluid within the star. We compute their unstable modes of oscillations driven by gravitational wave radiation and shear viscosity, modeling them as triaxial Riemann ellipsoids. We work in a perturbative regime, where the gravitational radiation reaction force is taken into account at 2.5-post-Newtonian order and find unstable modes with dissipation timescales $\gtrsim 1$ ms which are relevant to the transient state of a hypermassive remnant of a merger. We show that the secular instabilities are dominated by gravitational wave radiation. If the shear viscosity is included, it can increase the growth times or even stabilize the unstable modes, but it must have values several orders of magnitude larger than predicted for cold neutron stars.

gr-qc↗

arXiv2020

Relativistic finite temperature multifluid hydrodynamics in a neutron star from a variational principle

We develop a relativistic multifluid dynamics appropriate for describing neutron star cores at finite temperatures based on Carter's convective variational procedure. The model includes seven fluids, accounting for both normal and superfluid/superconducting neutrons and protons, leptons (electrons and muons) and entropy. The formulation is compared to the non-variational relativistic multifluid hydrodynamics of Gusakov and collaborators and shown to be equivalent. Vortex lines and flux tubes, mutual friction, vortex pinning, heat conduction and viscosity are incorporated into the model in steps after the basic hydrodynamics is described. The multifluid system is then considered at the mesoscopic scale where the currents around individual vortex lines and flux tubes are important, and this mesoscopic theory is averaged to determine the detailed vortex line/flux tube contributions to the macroscopic "effective" theory. This matching procedure is partially successful, though obtaining full agreement between the averaged mesoscopic and macroscopic effective theory requires discarding subdominant terms. The matching procedure allow us to interpret the magnetic $H$-field inside a neutron star in a way that is consistent with condensed matter physics literature, and to clarify the difference between this interpretation and that in previous astrophysical works.

astro-ph.HE↗