Simulating the Space Weather in the AU Mic System: Stellar Winds and Extreme Coronal Mass Ejections

TL;DR

This study models the stellar wind and coronal mass ejections of AU Mic, revealing extreme space weather conditions that threaten exoplanet atmospheres and providing insights into the system's dynamic space environment.

Contribution

The paper introduces 3D numerical models of AU Mic's stellar wind and CMEs, incorporating observational data to characterize the system's space weather environment.

Findings

Predicted sub-Alfvénic regions along exoplanet orbits.

High dynamic and magnetic pressures in quiescent conditions.

Harsh space weather during CME events threatening exoplanet atmospheres.

Abstract

Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the so-called "evaporation valley" in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by the high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation on this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic Coronal Mass Ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict an extreme space weather for AU Mic and its planets. This includes sub-Alfv\'enic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within $10^{2} - 10^{5}$ times the dynamic pressure experienced by the Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of the exoplanetary atmospheres (if any) in this system.