White dwarf debris disks, planetary remnants

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White dwarf stars can shred and vaporise any asteroids or rocky planets in their vicinity, if these small bodies are flung inward by gravitational interaction with a larger outer body. The debris disk reveals itself spectroscopically via double peaked emission lines. From the New Scientist article:

The spectra indicate that a disc containing calcium, magnesium, and iron gas is orbiting the white dwarf at a distance 100 times closer than Mercury’s orbit around the Sun (below right). At this distance, intense radiation from the white dwarf heats the gas to 5000 Kelvin. The spectra also show that the white dwarf’s atmosphere is enriched in magnesium. That indicates material from the disc is falling onto the star, since the star’s own surface gravity is so great that its own heavy elements should have already sunk towards its centre – and out of sight.

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In another, early postulation of a tidally destroyed asteroid calcium was found to be abundant in the white dwarf G29-38. There is now a diversity of up to eleven different metals observed, with carbon-to-silicon ratios consistent with bulk Earth material, implying rocky material made up of iron- and magnesium- rich silicates.
White_dwarf_wide_FINALMR

Observationally, disk material is sought using wide area surveys via detection of infrared excesses. Cool DZ white dwarfs, with metal lines, have been found in the Sloan Digital Sky Survey. Rather than originating in the interstellar medium, as was thought for many years, the source of the metal lines is rocky planetesimals. Since white dwarfs are the end point of evolution of the vast majority of stars, observations of metal-rich material associated with them give a robust lower limit on the number of stars like the Sun, as well as those rather more massive, which form planetary systems.
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Main image credit. Note this is a simulation of an accretion disk around a compact object, such as a white dwarf, rather than a bona-fide debris disk illustration.

Update: Detecting biomarkers via transits of white dwarfs?

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