Theoretical astrophysics and plasma physics at RPC

Abstract:

The spin–orbit misalignment of stellar-mass black hole (sBH) binaries provides important constraints on the formation channels of merging sBHs. Here, we study the role of secular spin–orbit resonance in the evolution of an sBH binary component around a supermassive BH (SMBH) in an AGN disk. We consider the sBH's spin precession due to the J2 moment introduced by a circum-sBH disk within the warping/breaking radius of the disk. We find that the sBH's spin–orbit misalignment (obliquity) can be excited via spin–orbit resonance between the sBH binary's orbital nodal precession and the sBH spin precession driven by a massive circum-sBH disk. Using an α-disk model with Bondi–Hoyle–Lyttleton accretion, the resonances typically occur for sBH binaries with semimajor axis of 1 au and at a distance of ∼1000 au around a 107 M⊙ SMBH. The spin–orbit resonances can lead to high sBH obliquities and a broad distribution of sBH binary spin–spin misalignments. However, we note that the Bondi–Hoyle–Lyttleton accretion is much higher than that of Eddington accretion, which typically results in spin precession being too low to trigger spin–orbit resonances. Thus, secular spin–orbit resonances can be quite rare for sBHs in AGN disks.

Abstract:

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emission. Using fully kinetic particle-in-cell simulations, we investigate how the guide-field strength and domain size affect the characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy γ _inj and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and γ _inj , which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to large-scale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. The Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide-field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide-field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emission in high-energy astrophysics, including black hole jets and pulsar wind nebulae

Abstract:

We investigate the effects of an oscillating gas filament on the dynamics of its embedded stellar clusters. Motivated by recent observational constraints, we model the host gas filament as a cylindrically symmetrical potential, and the star cluster as a Plummer sphere. In the model, the motion of the filament will produce star ejections from the cluster, leaving star cluster remnants that can be classified into four categories: (a) filament-associated clusters, which retain most of their particles (stars) inside the cluster and inside the filament; (b) destroyed clusters, where almost no stars are left inside the filament, and there is no surviving bound cluster; (c) ejected clusters, that leave almost no particles in the filament, since the cluster leaves the gas filament; and (d) transition clusters, corresponding to those clusters that remain in the filament, but that lose a significant fraction of particles due to ejections induced by filament oscillation. Our numerical investigation predicts that the Orion Nebula cluster is in the process of being ejected, after which it will most likely disperse into the field. This scenario is consistent with observations which indicate that the Orion Nebula cluster is expanding, and somewhat displaced from the integral-shaped filament ridgeline.

Abstract:

Starting from the equations of collisionless linear gyrokinetics for magnetised plasmas with an imposed inhomogeneous magnetic field, we present the first known analytical, closed-form solution for the resulting velocity-space integrals in the presence of resonances due to both parallel streaming and constant magnetic drifts. These integrals are written in terms of the well-known plasma dispersion function (Faddeeva & Terent'ev, Tables of Values of the Function w(z)=exp(−z2)(1+2i/ √ π ∫ z 0 exp(t2)dt) for Complex Argument, 1954. Gostekhizdat. English translation: Pergamon Press, 1961; Fried & Conte, The Plasma Dispersion Function, 1961. Academic Press), rendering the subsequent expressions simpler to treat analytically and more efficient to compute numerically. We demonstrate that our results converge to the well-known ones in the straight-magnetic-field and two-dimensional limits, and show good agreement with the numerical solver by Gürcan (J. Comput. Phys., vol. 269, 2014, p. 156). By way of example, we calculate the exact dispersion relation for a simple electrostatic, ion-temperature-gradient-driven instability, and compare it with approximate kinetic and fluid models.

Abstract:

Tidal dissipation plays an important role in the dynamical evolution of moons, planets, stars, and compact remnants. The interesting complexity originates from the interplay between the internal structure and external tidal forcing. Recent and upcoming observing missions of exoplanets and stars in the galaxy help to provide constraints on the physics of tidal dissipation. It is timely to develop new N-body codes, which allow for experimentation with various tidal models and numerical implementations. We present the open-source N-body code TIDYMESS, which stands for ‘TIdal DYnamics of Multibody ExtraSolar Systems’. This code implements a Creep deformation law for the bodies, parametrized by their fluid Love numbers and fluid relaxation times. Due to tidal and centrifugal deformations, we approximate the general shape of a body to be an ellipsoid. We calculate the associated gravitational field to quadruple order, from which we derive the gravitational accelerations and torques. The equations of motion for the orbits, spins and deformations are integrated directly using a fourth-order integration method based on a symplectic composition. We implement a novel integration method for the deformations, which allows for a time-step solely dependent on the orbits, and not on the spin periods or fluid relaxation times. This feature greatly speeds up the calculations, while also improving the consistency when comparing different tidal regimes. We demonstrate the capabilities and performance of TIDYMESS, particularly in the niche regime of parameter space where orbits are chaotic and tides become non-linear.