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Nearest, youngest star with a dusty debris disk found
But are there planets forming? Inference suggests there could be, but further observations will be required

| 04 March 2004

 



The dust disk surrounding the red dwarf star AU Mic, seen in optical scattered light. The central dark region is produced by an occulting spot suspended by four wires, masking the star. The dust disk observed near the boundary with the black mask approximately corresponds to the Kuiper Belt of asteroids in our solar system.
Paul Kalas/UC Berkeley, courtesy of Science Magazine

Berkeley astronomers have discovered the nearest and youngest star with a visible disk of dust that may be a nursery for planets.

The dim red dwarf star is a mere 33 light years away, close enough that the Hubble Space Telescope, or ground-based telescopes with adaptive optics to sharpen the image, should be able to see whether the dust disk contains clumps of matter that might turn into planets.

“Circumstellar disks are signposts for planet formation, and this is the nearest and youngest star where we directly observe light reflected from the dust produced by extrasolar comets and asteroids — i.e., the objects that could possibly form planets by accretion,” says Paul Kalas, assistant research astronomer at Berkeley and lead author of a paper reporting the discovery.

“We’re waiting for the summer and fall observing season to go back to the telescopes and study the properties of the disk in greater detail. But we expect everyone else to do the same thing; there will be lots of follow-up.”

The young M-type star, AU Microscopium (AU Mic), is about half the mass of the sun but only about 12 million years old, compared to the 4.6-billion-year age of the sun. The team of astronomers found the star while searching for dust disks around stars emitting larger-than-expected amounts of infrared radiation, indicative of a warm, glowing dust cloud.

Clues to planetary evolution
The image of AU Mic, obtained last October with the University of Hawaii’s 2.2-meter telescope atop Mauna Kea, shows an edge-on disk of dust stretching about 210 astronomical units from the central star — about seven times farther from the star than Neptune is from the sun. One astronomical unit, or AU, is the average distance from the Earth to the sun, about 93 million miles.

“When we see scattered infrared light around a star, the inference is that this is caused by dust grains replenished by comets and asteroid collisions,” Kalas says. Because 85 percent of all stars are M-type red dwarfs, the star provides clues to how the majority of planetary systems form and evolve.

Though other nearby stars provide indirect evidence for planets, images of debris disks around stars are rare. AU Mic is the closest dust disk directly imaged since the discovery 20 years ago of a dust disk around beta-Pictoris, a star about 2.5 times the mass of the sun and 65 light years away. Though the two stars are in opposite regions of the sky, they appear to have been formed at the same time and to be traveling together through the galaxy, Kalas says.

“These sister stars probably formed together in the same region of space in a moving group containing about 20 stars,” Kalas says. This represents an unprecedented opportunity to study stars formed under the same conditions, but of masses slightly larger and slightly smaller than the sun. Kalas adds that theorists are also excited by the opportunity to understand how planetary systems evolve differently around high-mass stars like beta-Pictoris and low-mass stars like AU Mic.

The pictures of AU Mic were obtained by blocking glare from the star with a coronagraph like that used to view the sun’s outer atmosphere, or corona. The eclipsing disk on the University of Hawaii’s 2.2-meter telescope blocked observers’ view of everything around the star out to about 50 AU. At this distance in our solar system, only the Kuiper Belt of asteroids and the more distant Oort cloud, the source of comets, would be visible.

Kalas says that sharper images from the ground or space should show structures as close as 5 AU, which means a Jupiter-like planet or lump in the dusty disk would be visible if present.

“With the adaptive optics on the Lick 120-inch telescope or the Keck 10-meter telescopes, or with the Hubble Space Telescope, we can improve the sharpness by 10 to 100 times,” Kalas says.

A paper announcing the discovery, recently published online in Science Express, will appear in the printed edition of the journal in March. Coauthors with Kalas are Brenda Matthews, a post-doctoral researcher with Berkeley’s Radio Astronomy Laboratory, and astronomer Michael Liu of the University of Hawaii. Kalas also is affiliated with the Center for Adaptive Optics at UC Santa Cruz. The research was supported by the NASA Origins Program and the National Science Foundation’s Center for Adaptive Optics.