White Dwarf Has Rare Ring System After Ripping An Asteroid Apart

Shown is an artist's impression of the star and its ring. Manser et al. 2015

A team of scientists has observed a dying star shredding an asteroid to bits and forming a glowing disk of debris around itself. This is the first time a star has been caught in the act of forming a ring system.

The team from the University of Warwick observed the white dwarf designated SDSS1228+1040, which is the remnant of a much larger star, for 12 years. This allowed them to detect detailed structures within the debris disk. The ring gap around the white dwarf is 700,000 kilometers (435,000 miles) across and the ring itself has a mass comparable to a large asteroid.  

The white dwarf is only 180 million years old and it is 463 light years from Earth. It is a very compact object, packing three-quarters of the Sun’s mass into one percent of its radius. 

The ring is composed of gas and dust with possibly some sizeable chunks, although more observations are needed to confirm this. It is made mostly of oxygen, iron, magnesium, and calcium, a composition similar to the basalt-rich rocky asteroids of the Solar System.

The technique used in the study, called Doppler Tomography, is akin to a CT scan. But instead of having detectors moving around a patient, the team had to observe the disk rotating. The observation was mostly carried out using  the European Southern Observatory's (ESO) Very Large Telescope

This image of the debris disk around SDSS1228+1040 was made from observations taken over twelve years. The application of Doppler Tomography results in an image of the velocities within the disk, which has an `inside-out’ structure; gas closer to the white dwarf appears further. The two dashed circles illustrate velocities at 0.64 and 0.2 times the radius of the Sun. Manser et al. 2015

This particular object is a rare finding. “We see dust disks around two percent of all white dwarfs, but the stars which also have gas like the one we studied in this paper are much rarer,” Professor Boris Gänsicke, co-author on the study, told IFLScience. “They are about one in a thousand.”  

The study could also help astronomers understand how planetary systems evolve, and if they are similar to our own.

“These systems, in general, can tell us about the composition of the asteroids we see orbiting the white dwarfs and that we see eventually ending up on the surface and in the atmosphere of the stars,” added Gänsicke. “If we look at the element abundance in the disk, we can discover what these asteroids were made of, and that can feedback our understanding of planet formation as a whole. It is very similar to the work planetary scientists are doing in the Solar System where they look at meteorites to study the composition of the primordial Solar System.”

The study is published in the Monthly Notices of the Royal Astronomical Society.

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