Grain alignment and rotational disruption by radiative torques in exoplanet atmospheres

TL;DR

This paper investigates how radiative torques align dust grains in hot Jupiter atmospheres and how rotational disruption affects their size distribution, impacting observable properties and high-altitude cloud formation.

Contribution

It introduces a detailed analysis of grain alignment and disruption mechanisms in exoplanet atmospheres, highlighting the roles of magnetic fields and radiation in these processes.

Findings

Silicate grains align with magnetic fields (B-RAT), while carbonaceous grains align with radiation (k-RAT).

Large grains are disrupted into smaller ones by RATD at high altitudes.

Disruption of large grains may explain high-altitude clouds and super-puff 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 is a promising method to study magnetic fields of exoplanets. Moreover, the grain size distribution plays an important role in physical and chemical processes in the atmospheres, which is rather uncertain in atmospheres. In this paper, we first study 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 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 $r<2R_{\rm p}$ with $R_{\rm p}$ being the planet radius, only large grains can be aligned, but tiny grains of $a\sim 0.01\mu$m can be aligned at a high altitude of $r>3R_{\rm p}$. We then study rotational disruption of dust grains by the RAdiative Torque Disruption (RATD) mechanism. We find that large grains can be disrupted by RATD into smaller sizes. Grains of high tensile strength are disrupted at an altitude of $r>3R_{\rm p}$, but weak grains can be disrupted at a lower altitude. We suggest that the disruption of large grains into smaller ones can facilitate dust clouds to escape to high altitudes due to lower gravity and may explain the presence of high-altitude clouds in hot Jupiter as well as super-puff atmospheres.