Tidal debris orbiting white dwarfs is limited by a kilometer-size barrier

White dwarfs often possess disks of planetary debris created when rocky bodies are torn apart by tidal forces. New calculations show that these fragments are not infinitely small, but are limited by a kilometer-size barrier due to internal cohesive forces. This finding changes how the evolution of these debris disks is modeled, highlighting the role of collisions and material strength in shaping planetary remnants.

Star system: WD 0145+234 | Distance from the Sun: 96 light years

When a star like our Sun reaches the end of its life, it sheds its outer layers and becomes a dense, hot core known as a white dwarf. These remnants often host disks of rocky debris, which are thought to form when planets or asteroids wander too close and are torn apart by the white dwarf's intense gravity. Previous models often assumed these debris bodies acted like loose collections of rubble with no internal strength, breaking down into tiny particles almost immediately. However, even loose rubble piles possess a small amount of internal cohesiveness caused by forces acting at the atomic level, known as Van der Waals forces.

Accounting for this internal strength reveals that the fragments produced during tidal breakup cannot be arbitrarily small. By calculating the breakup limits for materials including water ice, iron, and rock, it is clear that a maximum fragment size exists. This limit, identified as the kilometer-size barrier, means that the pieces of debris are typically between 0.1 and 1 kilometer in size. Because most of the mass in these disks resides within these larger fragments, the way the disk evolves over time must be reconsidered.

This size restriction indicates that before forces like radiation drag can move or shape the dust in the disk, the debris must first undergo significant collisional grinding. This process breaks the larger kilometer-sized chunks into finer dust. Furthermore, the internal strength of these objects causes the debris to remain more closely confined in their orbits around the white dwarf than earlier, strengthless models suggested. These findings provide a more accurate framework for understanding the lifecycle of material found in systems orbiting white dwarfs.

Key findings
Internal cohesive forces prevent debris fragments from breaking down smaller than a specific limit.
The maximum fragment size for rubble piles around white dwarfs is typically 0.1 to 1 kilometer.
Most of the mass in white dwarf debris disks is contained within these kilometer-sized fragments.
Collisional grinding is a necessary precursor to radiation-driven disk evolution.
Debris fragments with internal strength remain more narrowly confined in their orbits.

Scientific study

Size Limits on Tidal Debris around White Dwarfs: The Kilometer-size Barrier (external link)