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Reshaping views of how young stars evolve.

Reshaping views of how young stars evolve

When young stars blast their own matter into space, the mass rushes outward in spherical, concentric shells, carrying roughly the same amount of material in every direction. At least that's what theory predicts. But British and Canadian astronomers say they may have found an exception to the rule.

Their radio mapping of one of the brightest stars in the Milky Way indicates the object has jettisoned mass mainly in one direction, at several times the presumed rate of other young stars.

Using a sensitive array of radio telescopes at the Nuffield Radio Astronomy Laboratories in Macclesfield, England, Richard J. Davis and his co-workers monitored the activity of AS431, a member of the Wolf-Rayet group of young, hot stars. They observed two distinct, unusual radio sources--each as large as the entire solar system--associated with the star. One source coincides with the star's optical image, and the other, warmer source indicates the presence of a companion object, Davis and his colleagues report in the August 10 NATURE. They speculate that a collision of ejected mass from AS431 with the star's apparent companion may explain the unexpected radio emissions.

The warmer temperature of the companion-related radio source, the researchers find, is double the temperature associated with stars that shed mass in spherical layers. Surprisingly intense X-rays previously detected by other astronomers may emanate from this source. Davis' team asserts that a unidirectional blast of mass from AS431 -- energetic enough to set up shock waves between the young star and its apparent companion -- is a plausible explanation for the radio and X-ray wave activity in and around the star, located approximately 6,000 light-years from Earth.

"Usually, a normal star blows off its outer regions dramatically but steadily in a spherical shell. [But] here there is an asymmetry, a [directed] shock wave the size of the solar system, much larger than the star itself," says Davis, who collaborated with other astronomers at Nuffield; England's University of Cambridge; the Lancashire Polytechnic in Preston, England; the British Antarctic Survey in Cambridge; and the University of Calgary in Alberta.

Validating the team's theory of a unidirectional ejection will require future observations. If the warmer source emits its radio waves as a result of collision with mass ejected from AS431, the intensity of its signals should diminish or even fizzle out over a few years.

The findings, if confirmed, would have several implications for other young stars, Davis says. If newly formed stars lose mass in different ways and more rapidly than theorized, their lifetimes may differ from calculated values, because less massive stars generally burn more slowly and last longer. In addition, he says, if stars eject unexpectedly large amounts of matter that later cools, this could account for some of the so-called "dark matter" in the universe -- mass believed present but hidden from view because it does not radiate at observed wavelengths.
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Author:Cowen, R.
Publication:Science News
Date:Aug 12, 1989
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