The findings revisit the interpretation of the rocky dust rings that surround some white dwarfs, and raise an intriguing question: if the gravity of dead stars tears asteroids into large rocks, what then grinds these rocks into the dust that telescopes observe?
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Previous models relied on the assumption that asteroids composed of debris piles lack internal forces binding their grains together. However, real asteroids are not completely disintegrating; even small grains are attracted by weak forces known as van der Waals forces, which are sufficient to give the debris a degree of cohesion.
Lead researcher Jordan Sticklough said, "Nothing is completely incoherent," explaining that the forces between grains give comets and asteroids tens of times weaker resistance to tearing than modern ice does.
The team incorporated these forces into their calculations for the disintegration of asteroids near white dwarfs, taking into account material density and distance from the star. The results showed that most of the mass remains in fragments ranging from 100 to 1000 meters in diameter, rather than breaking down directly into microscopic dust grains.
The researchers called this result the "kilometer barrier," referring to the size determined by the resistance of the rubble to tearing due to gravity.
A lost stage between crash and dust
The problem is that the dust astronomers observe around white dwarfs is much smaller than these fragments. Therefore, tidal and gravitational forces alone are insufficient to explain the formation of the rings; additional processes are needed for the fragmented rock, such as repeated collisions or the disintegration caused by its rapid rotation.
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