Glowing rings of a dying star

Glowing debris ring around white dwarf star

Researchers have found the remains of a planetary system orbiting a white dwarf star.

Researchers from the University of Warwick have acquired the first direct image of a debris disc in orbit around the core of a burnt out star. The white dwarf star, SDSS1228+1040, lies about 463 lightyears away in the constellation of Virgo.

Although rare, debris discs around white dwarfs have been known about for around two decades, but this is the first time we’ve seen an actual image of one. Data from the Sloan Digital Sky Survey (SDSS), together with images from the Herschel and Isaac Newton Telescopes in La Palma and ESO’s Very Large Telescope in Chile were used by a team led by Christopher Manser at the university’s Astrophysics Group. Out of 406 white dwarfs studied using the SDSS data, SDSS1228+1040 was the only one of its kind discovered.

A technique known as Doppler Tomography was used to create this new image – the culmination of 12 years of combined data using a series of scans similar to Computed Tomography (CT scans). The disc is gradually rotating and was repeatedly scanned from slowly shifting angles before being combined using advance computer software.

Debris ring around a white dwarf starThe metal-rich ring system orbiting SDSS1228+1040 is thought to be the remains of a 50 km wide asteroid which has been ripped apart by the intense gravity of the once massive star. Interactions between dust particles in the disc have created a glowing ring of gas, which is being illuminated by the ultraviolet light emitting from the white dwarf. Imagine the Saturnian ring system but much, much bigger. The central hole in the doughnut-shaped ring is about 700,000 km wide – half the diameter of the Sun. The star is seven times smaller than Saturn, but weighs 2,500 times more!

The main sequence star responsible for the white dwarf we see today would have  matured into a red giant before being stripped down to its inner core. During the red giant stage, any planets in close orbits would have been vaporised or flung out into wider orbits as the star expanded. As the resulting planetary nebula dissipated, it’s likely that a pre-existing planet dislodged an asteroid from its stable orbit, bringing it into the Roche limit  – an area around a star or planet where intense gravity causes tidal forces to rip the unfortunate object apart (Saturn’s rings are a fine example of this). The elements, including calcium and magnesium, found in the debris ring are consistent with an asteroid – probably one of several in orbit around the star.

The progenitor of this slowly cooling stellar ember was possibly about four or five times the mass of the Sun and is thought to have had a relatively short lifespan of about 70 million years. Until recently, many astronomers thought that planetary systems were unable to evolve around such short-lived, massive stars, but the findings of this new research indicate otherwise.

Results of this research were published in the Monthly Notices of the Royal Astronomical Association in November 2015. The authors suggest that what is being observed at SDSS1228+1040 could offer clues to the fate of our own Solar System, five to eight billion years in the future.


© Melanie Davies 2015

 

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