Atmospheric escape in hot Jupiters under sub-Alfv\'enic interactions

TL;DR

This study uses 3D simulations to explore how magnetic fields influence atmospheric escape in hot Jupiters within sub-Alfvénic stellar wind environments, revealing new escape behaviors and potential observational diagnostics.

Contribution

It introduces the first detailed 3D MHD models of hot Jupiter atmospheric escape under sub-Alfvénic conditions, highlighting magnetic confinement effects and novel Lyman-$\\alpha$ transit signatures.

Findings

Escape rate increases with planetary magnetic field strength.

Only one significant polar outflow observed, unlike super-Alfvénic models.

Lyman-$\alpha$ absorption ratios can diagnose stellar wind and magnetic field properties.

Abstract

Hot Jupiters might reside inside the Alfv\'en surface of their host star wind, where the stellar wind is dominated by magnetic energy. The implications of such a sub-Alfv\'enic environment for atmospheric escape are not fully understood. Here, we employ 3-D radiation-magnetohydrodynamic simulations and Lyman-$\alpha$ transit calculations to investigate atmospheric escape properties of magnetised hot Jupiters. By varying the planetary magnetic field strength ($B_p$) and obliquity, we find that the structure of the outflowing atmosphere transitions from a magnetically unconfined regime, where a tail of material streams from the nightside of the planet, to a magnetically confined regime, where material escapes through the polar regions. Notably, we find an increase in the planet escape rate with $B_p$ in both regimes, with a local decrease when the planet transitions from the unconfined to the confined regime. Contrary to super-Alfv\'enic interactions, which predicted two polar outflows from the planet, our sub-Alfv\'enic models show only one significant polar outflow. In the opposing pole, the planetary field lines connect to the star. Finally, our synthetic Ly-$\alpha$ transits show that both the red-wing and blue-wing absorptions increase with $B_p$. Furthermore, there is a degeneracy between $B_p$ and the stellar wind mass-loss rate when considering absorption of individual Lyman-$\alpha$ wings. This degeneracy can be broken by considering the ratio between the blue-wing and the red-wing absorptions, as stronger stellar winds result in higher blue-to-red absorption ratios. We show that, by using the absorption ratios, Lyman-$\alpha$ transits can probe stellar wind properties and exoplanetary magnetic fields.