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Hubble Discovers Lopsided Star


Dust and debris parade in an extremely misshapen ring around the young star, HD 15115. The lopsided disk of material encircling the star was discovered by astronomers using the Hubble Space Telescope. Follow-up observations at W.M. Keck Observatory investigated the odd disk further.

Image of the lopsided star

Image above: This NASA Hubble Space Telescope image shows a lopsided debris disk around the young star HD 15115. The disk, seen edge-on, is the dense blue line extending from the star to the upper right and lower left of the image. Astronomers think the disk's odd imbalanced look is caused by dust particles following a highly elliptical orbit around the star, which is slightly brighter than the Sun. Astronomers used a mask on Hubble's Advanced Camera for Surveys to block out the bright starlight so they could see the dim disk.

“It is so unusual that we didn’t even think it was real in the Hubble data,” says Paul Kalas of the University of California at Berkeley. “That’s why we went to Keck, because we needed confirmation.”

The disk’s shape resembles a pizza whose cheese and toppings have all slid to one side. Astronomers currently are tossing around two possible causes for this imbalance. Perhaps the gravity of a neighboring star warped the disk, or maybe a planet within the system swept up the debris.

HD 15115 is a member of the Beta Pictoris Moving Group, a caravan of about 30 stars traveling together through space. Astronomers think stars in these stellar convoys share a common birthplace and age. In the case of the Beta Pictoris Moving Group, the stars are about 12 million years old.

One theory explaining the star’s lopsided disk is that a companion star mangled it. A vagrant star, HIP 12545, may have perturbed the disk. As the star drifted by, its gravity could have pulled the material into its highly asymmetrical shape. HIP 12545 is about 10 light-years away from HD 15115. “It’s not terribly close,” Kalas says. “But it’s suspiciously close.”

Another theory involves possible planetary influence. Protoplanetary bodies smash into one another in debris disks leaving behind their dusty, fragmented remains. A planet could have been knocked into a racetrack-shaped orbit during planetary upheaval. If this planet was fairly close to the star, it would have an influence over the debris disk just like Jupiter’s gravity affects the asteroid belt. It could have slowly disturbed the dusty particles into a similarly extreme orbit.

“The lopsided disk presents a host of new challenges for theorists,” Kalas says. “How do you make a needle-like dust disk around a star and how long can it survive in this configuration? Was there a violent upheaval of a relatively normal disk by a nemesis star in the recent past, or is the disk simply following the persistent tug of an inner planet that has a lopsided orbit?”

Kalas believes that the research of HD 15115 will prompt a wealth of follow-up observations. This fall he will investigate further HP 12545 to see if it has a dusty disk.

Kari Reitin
Space Telescope Science Institute

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Last Updated: July 19, 2007
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