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Sublimation Could Play Role in Forming White Dwarf Debris Disks

Jordan Steckloff DPS press conference 2020

Screen shot from Jordan Steckloff’s DPS press conference presentation.

 

PSI Research Scientist Jordan Steckloff discussed his abstract “The Physical Processes Restricting Dusty Debris Disks to Cooler White Dwarfs” during a press conference at the virtual 52nd annual meeting of the American Astronomical Society’s Division for Planetary Sciences. 

Disks of dusty debris have been found around about 4 percent of white dwarf stars, the glowing ember of  a dead star’s core. Since any such debris would have been destroyed when the star went through it red giant phase, prior to becoming a white dwarf, any observed debris would have had to have migrated inward from greater distances. 

This isn’t new; there is a two-step model to explain how these disks form. First, an asteroid is scattered into a stargrazing orbit, where is passes so close to the white dwarf that tidal forces break the asteroid into dust. Second, radiation forces circularize the dust orbits into a dusty debris disk. 

However, this process seemed inconsistent with white dwarf observations; if dusty debris disks form from inwardly scattered asteroids and radiative forces, then one would expect young white dwarfs to predominantly host dusty debris disks. Young white dwarfs are more likely to host systems that were destabilized by the red giant phase, and are brighter (radiative forces would circularize orbits more quickly). However dusty debris disks are exclusively found around older white dwarfs that have had time to cool down to approximately 27,000 degrees Kelvin (48,140 degrees Fahrenheit) or below. We wanted to understand what process is causing this disagreement. 

We found that there is another process controlling the formation of dusty debris disks: sublimation. To form a dusty debris disk, an asteroid must pass close enough to the white dwarf to be broken apart by its tidal forces. However, if it gets too close to the star, then the dust itself will vaporize. Thus, the asteroid must not only pass inside of the tidal disruption limit, but also outside of this sublimation radius. We learned that this isn’t possible for younger white dwarfs. The sublimation radius lies outside of the tidal disruption limit; any asteroid that passes inside of the tidal disruption limit will rapidly vaporize or sublimate away. However, these two conditions can be met once the white dwarf cools to below approximately 27,000 degrees Kelvin. 

Jordan Steckloff DPS press conference 2020

Screen shot from Jordan Steckloff’s DPS press conference presentation.

Nov. 22, 2020

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