Celestial sunflowers -- Survival of rings around small planetary bodies under solar radiation pressure

Abstract

Context: Rings around giant planets are a common feature of the solar system. Even though solar radiation pressure is known to destabilize rings by exciting the orbital eccentricity of its particles, the Centaur Chariklo (and possibly Chiron), the dwarf planet Haumea, and trans-Neptunian object Quaoar also host rings of solid material. Aims: We explore the dynamical evolution of rings around spherical Chariklo and Haumea analogs, assuming different particle sizes and tilt angles with respect to the planetary orbital plane of the ring. Methods: The ring dynamics were studied using a GPU-based N-body integrator with an 8th-order Hermite scheme for several thousand years, corresponding to 10 solar orbits. The simulations took into account the gravitational effects of the planet and the Sun, radiation pressure, and the shadow cast by the planet. Results: Two families of rings have been identified depending on the ring tilt angle. Slightly tilted rings (<=40 deg) are unstable under a critical particle size. Highly tilted rings (>=50 deg), however, show instability only for a range of particle sizes that spans 1-10 times the critical size. The planetary shadow reduces the critical size by a factor of five and extends the instability region to 0.1-10 times this newly identified critical size. Conclusions: The stabilization of highly inclined rings occurs because the plane of the ring is forced to be perpendicular to the Solar radiation. As a result, the plane of the ring rotates as the planetary bodies revolves: always facing the sun, like a celestial sunflower. Rings which are closely aligned to the orbital plane of the host planet, such as Haumea and Quaoar, presumably consist of particles with a size at least 1-4 um. However, particles in the rings which are highly tilted, like that around Chariklo and Chiron, should consist of particles <=2.5-15 um or >=60-300 um.