Stellar Winds and Dust Avalanches in the AU Mic Debris Disk

Abstract

We explain the fast-moving, ripple-like features in the edge-on debris disk orbiting the young M dwarf AU Mic. The bright features are clouds of sub-micron dust repelled by the host star's wind. The clouds are produced by avalanches: radial outflows of dust that gain exponentially more mass as they shatter background disk particles in collisional chain reactions. The avalanches are triggered from a region a few AU across -- the "avalanche zone" -- located on AU Mic's primary "birth" ring, at a true distance of $\sim$35 AU from the star but at a projected distance more than a factor of 10 smaller: the avalanche zone sits directly along the line of sight to the star, on the side of the ring nearest Earth, launching clouds that disk rotation sends wholly to the southeast, as observed. The avalanche zone marks where the primary ring intersects a secondary ring of debris left by the catastrophic disruption of a progenitor up to Varuna in size, less than tens of thousands of years ago. Only where the rings intersect are particle collisions sufficiently violent to spawn the sub-micron dust needed to seed the avalanches. We show that this picture works quantitatively, reproducing the masses, sizes, and velocities of the observed escaping clouds. The Lorentz force exerted by the wind's magnetic field, whose polarity reverses periodically according to the stellar magnetic cycle, promises to explain the observed vertical undulations. The timescale between avalanches, about 10 yr, might be set by time variability of the wind mass-loss rate or, more speculatively, by some self-regulating limit cycle.

Accepted to ApJ

References in corpus (7)

• On the age of the $\beta$ Pictoris moving group
• Probing for Exoplanets Hiding in Dusty Debris Disks: Disk Imaging, Characterization, and Exploration with HST/STIS Multi-Roll Coronagraphy
• Long-Term Collisional Evolution of Debris Disks
• Debris from giant impacts between planetary embryos at large orbital radii
• Large Impacts around a Solar Analog Star in the Era of Terrestrial Planet Formation
• Collisional dust avalanches in debris discs
• Evidence for photometric activity cycles in 3203 Kepler stars

Cited by in corpus (11)

• A planet within the debris disk around the pre-main-sequence star AU Microscopii
• Investigating the young AU~Mic system with SPIRou: large-scale stellar magnetic field and close-in planet mass
• Planet-induced radio emission from the coronae of M dwarfs: the case of Prox Cen and AU Mic
• The Mass of Stirring Bodies in the AU Mic Debris Disk Inferred from Resolved Vertical Structure
• Extreme Debris Disk Variability -- Exploring the Diverse Outcomes of Large Asteroid Impacts During the Era of Terrestrial Planet Formation
• The dichotomy of atmospheric escape in AU Mic b
• Expelled grains from an unseen parent body around AU Mic
• A Layered Debris Disk around M Star TWA 7 in Scattered Light
• Giant Impacts and Debris Disk Morphology
• Multiwavelength Vertical Structure in the AU Mic Debris Disk: Characterizing the Collisional Cascade
• ALMA's view of the M-dwarf GSC 07396-00759's edge-on debris disc: AU Mic's coeval twin