Case of the misshapen disk

From Austin, Texas, at a meeting of the American Astronomical Society

DEFORMED DISK. The swept-back, asymmetrical debris disk around the star HD 61005 suggests it collided with a dense gas cloud. Hines, G. Schneider, NASA

Debris disks surrounding stars are signposts that planet formation has already begun. The material in such disks is continually replenished as asteroids or other rocky debris—leftovers from the planet-making process—collides and produces fresh dust. Most debris disks are uniformly distributed around their star.

Not so the disk surrounding the star HD 61005. As observed with a near-infrared camera on the Hubble Space Telescope, part of the disk appears to be swept back behind the star.

The most plausible explanation for the misshapen disk is that the roughly 100-million-year-old star has plowed into a higher-density gas cloud, says Dean Hines of the Space Science Institute in Boulder, Colo. Moving at about 10 kilometers per second relative to the interstellar medium, HD 61005 would collide with enough oomph to bend its disk and blow dust grains backward, he adds. The very existence of a dense gas cloud in the star’s vicinity is a surprise, however, because the region, only 112 light-years from Earth, had been cleared of material by a relatively recent supernova explosion.

A simple color test could determine whether the collisional model is correct, says Paul Kalas of the University of California, Berkeley. In that model, the swept-back part of the disk would contain a higher proportion of tiny grains because they are more easily pushed than larger grains. Because small grains reflect a greater amount of blue light, visible-light images of the deformed disk, recorded by Hubble but not yet analyzed, ought to have a bluish tinge, he notes.

The collision isn’t likely to have harmed any planets that might already have formed close to HD 61005, Kalas adds. But if the star has an outer reservoir of comets and other icy bodies, similar to our solar system’s Kuiper belt, the violent interaction might partially deplete this frigid population.

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