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New evidence of dust rings around stars.

Astronomers reported new evidence this week that some young stars in the Milky Way have disks of hot gas and dust similar to the one believed to have once encircled the sun. And just as scientists speculate that clumps of material from the sun's disk formed the solar system's planets, the disks surrounding the young stars may one day give birth to planets orbiting these Milky Way bodies, the researchers say.

Vilppu Piirola of the University of Helsinki, Finland, and his colleagues deduced the presence of disks by analyzing the polarization of near-infrared light from the regions surrounding two youthful stars--V376 Cas and V633 Cas--located some 2,000 light-years from Earth in the constellation Cassiopeia. Visible-light images from the Palomar Observatory, near Escondido, Calif., had already demonstrated that a cocoon of light-obscuring dust surrounds each star, but these images lacked the resolution to detect flattened disks.

Examining the pattern of polarization --the direction in which the electric field of a light wave vibrates as the wave heads toward an observer -- in the new infrared images, Piirola and his colleagues probed the environment of the two stars more deeply. Dust particles with a diameter of about 1 micron polarize near-infrared light extremely well, and the researchers found that infrared light from large areas around each star was indeed highly polarized. But they also found a relative absence of polarized light from a flattened, disk-shaped region surrounding each star. Piirola and his co-workers attribute the reduced polarization to a thick disk of dust in which each tiny particle polarizes light in a different direction, effectively canceling out any net polarization. They report their work in the Oct. 1 NATURE.

Using the 2.5-meter Nordic Optical Telescope in the Canary Islands, Piirola and his team resolved structures as small as 100 astronomical units across (100 times the Earth-sun distance) - about twice the sharpness achieved by other researchers. He adds that the disks inferred in the present study are much thicker and more massive than the directly imaged disk around the older, nearby star Beta Pictoris.

These and other findings suggest that V376 Cas and V633 Cas -- just a few hundred thousand years old -- are still forming their circumstellar disks from the dust cloud in which they were born. In contrast, Beta Pictoris has evolved for several hundred million years, and its thinner disk, composed of larger particles, may represent leftover material from planets that have already formed.

In a separate study, scientists have found evidence that among a class of intermediate-mass stars that includes V376 Cas and V633 Cas, the largest members form the biggest disks and do so the most rapidly. The team, led by Lynne A. Hillenbrand and Stephen E. Strom of the University of Massachusetts at Amherst, presents its results in the Oct. 1 ASTROPHYSICAL JOURNAL.
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Title Annotation:similar to material that once surrounded the sun
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Oct 3, 1992
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