Shredded Asteroids – bet you can't eat three

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OK this may be a bit late but I thought I’d write about a story that got a bit of press last week. It’s based on this article by Jay Farihi and collaborators. Put simply most white dwarfs (the dead remains of stars, similar in mass to the Sun but the size of the Earth) should have atmospheres that are pure Hydrogen or pure Hellium. This is because the photosphere (the bit of the star the light is emitted from, the visible surface of the star if you like) isn’t very deep within the star. It’s thought that metals like Calcium will have sunk below this photosphere so there shouldn’t be any sign of these elements when these objects are studied. However some of these objects do show signatures of metals.

The question is, where did these come from? The authors look at two competing models, one where the white dwarfs get their metals from clouds of interstellar dust (clouds of dust that sits in the space between stars) and one where they get them from shredded asteroids. In the first model the white dwarfs move through space, collide with a dust cloud and gravity sucks some of the dust onto their surface. In the latter an asteroid in orbit around the white dwarf comes too close to it and is shredded by the strength of its gravity. Both cases should produce very different signatures when these white dwarfs are studied in mid-infrared light (about one tenth of the wavelength of visible light). Observations by the authors using the Spitzer space telescope combined with other observations suggest that some of these white dwarfs have signatures associated with disks of dust around them. This suggests an asteroid was shredded by the white dwarf’s gravity and formed a disk around it. All but a couple show signatures that are incompatible with sucking in interstellar dust. Hence it’s probable that these objects got their metals from sucking in dust that was made from shredded asteroids.

The final question is, how did the asteroids get close enough to be shredded. Surely they should just continue on their orbits like they do in the solar system. Well they authors speculate that this could be due to planets around the white dwarf. One of the these could give the asteroids a gravitational tug that could end up with them being flung towards the white dwarf, close enough to be shredded.