Grain Alignment and Rotational Disruption by Radiative Torques in Exoplanet Atmospheres

Abstract

Dust clouds are ubiquitous in the atmospheres of hot Jupiters and affect their observable properties. The alignment of dust grains in the clouds and resulting dust polarization provide a promising way to study the magnetic fields of exoplanets. Moreover, the grain size distribution plays an important role in the physical and chemical processes in the atmospheres, which are rather uncertain. In this paper, we first study the grain alignment of dust grains in the atmospheres of hot Jupiters by radiative torques (RATs). We find that silicate grains can be aligned by RATs with the magnetic fields (B − RAT) due to the strong magnetic fields of hot Jupiters, but carbonaceous grains of diamagnetic material tend to be aligned with the radiation direction (k − RAT). At a low altitude of r < 2R p, where R p is the planet radius, only large grains can be aligned, but tiny grains of a ∼ 0.01 μm can be aligned at a high altitude of r > 3R p. We then study the rotational disruption of dust grains by the RAT disruption (RAT-D) mechanism. We find that large grains can be disrupted by RAT-D into smaller sizes. Grains of high tensile strength are disrupted at an altitude of r > 3R p, but grains of low tensile strength can be disrupted at a lower altitude. We suggest that the disruption of large grains into smaller ones can facilitate dust clouds escaping to high altitudes due to lower gravity and may explain the presence of high-altitude clouds in hot Jupiters, as well as superpuff atmospheres.