Internal structures and circular orbits for test particles

TL;DR

This paper investigates how the internal structure of test particles, modeled with pole-dipole-quadrupole approximation, influences their stable circular orbits around Kerr black holes, revealing structure-dependent variations in orbit characteristics.

Contribution

It explicitly derives equations of motion considering quadrupole moments and analyzes their effects on orbital parameters in Kerr spacetime.

Findings

Inner stable orbit quantities increase with quadrupole effects.

Outer stable orbit radius decreases while other parameters increase.

Quadrupole structure significantly impacts orbital dynamics.

Abstract

We explore how the internal structure of a test particle affects its equatorial stable circular orbits around the Kerr black hole with or without a cosmological constant. To this end, we first explicitly write equations of motion for a test particle in the pole-dipole-quadrupole approximation specifying the quadrupole momentum tensor to a spin-induced model. Then we calculate characteristic quantities -- radius, angular momentum, energy, angular velocity, and impact parameter -- for the particles on the stable circular orbits. Once the pole-dipole-quadrupole approximation is taken, we find that for a particle on an innermost stable circular orbit, all characteristic quantities, except the angular velocity, become greater relative to the pole-dipole case. In contrast, for a particle on an outermost stable circular orbit, which only exists in the case of the spacetime background being asymptotically de Sitter, it is the radius that becomes smaller while all other quantities become greater.