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Reconsidering Interstellar Dust And Heavy Elements

University of Chicago astronomers have peered into the cosmic recycling bin of a neighboring galaxy. What they found inside will force astronomers to reconsider some of their ideas about interstellar dust and the buildup of heavy elements in the universe.

This is the statement of the University of Chicago's Daniel Welty, who presented the findings Monday (June 4) at the 198th meeting of the American Astronomical Society in Pasadena, Calif.

The study may provide new insights into the evolution of galaxies and the creation of heavy elements over a period of time that stretches as far back as 1 billion years after the big bang.

Only the lightest elements, such as hydrogen and helium, were created during the Big Bang. The heavy elements that were essential to the formation of Earth and other rocky planets, including silicon, iron, nickel and zinc, were created much later, in exploding stars.

Silicon, for example, is manufactured in explosions of massive stars that have lifetimes of millions of years. Iron is produced primarily in explosions of lighter, sunlike stars that reach the end of their lives after a billion years or more.

Before the Chicago study, it appeared that more silicon than iron was produced early in the lifetimes of young, distant galaxies, as is thought to have been the case for the Milky Way. Now, however, it seems possible that silicon and iron may have been produced in comparable amounts in those distant galaxies.

"It's a piece of the puzzle of trying to understand the build-up of the heavy elements-by establishing what the behavior is locally in interstellar matter and then using that information to interpret more distant galaxies that are much earlier in their evolutionary state," Welty said. His co-authors are Chicago's Lewis Hobbs and Donald York, James Lauroesch at Northwestern University, and Chris Blades at the Space Telescope Science Institute.

Using the Hubble Space Telescope's imaging spectrograph, Welty and his colleagues probed the seemingly empty space between the stars of the Small Magellanic Cloud, a small galaxy on the far outskirts of the Milky Way, for hints of the cosmic recycling process. The wispy ashes of dead stars that collect in this space as gas and dust provide both a record of previous generations of stars and the raw material for the creation of new stars.

A technique called absorption line spectroscopy enables scientists to determine what elements exist and in what quantities in interstellar gas clouds. But the composition of the gas in the clouds tells only part of the story. Some elements appear to be largely missing from the gas, and to be locked in solid dust grains instead.

Because it is diffucult to measure the composition of the dust directly, astronomers generally have assumed that the overall ratios of the elements and the general constituents of the mixture of dust grains were largely the same from one interstellar cloud to another. They had no reason to assume otherwise, based on previous observations made of interstellar clouds in the Milky Way.

But now Welty and his colleagues have observed that the relative proportions of some elements in the gas and dust in the Small Magellanic Cloud are different from those typically found in Milky Way clouds.

Welty's team took measurements along a line of sight between Earth and the star Sk 155 in the Small Magellanic Cloud, approximately 200,000 light years away. They use these observations of the Small Magellanic Cloud to help understand the evolution of heavy elements in younger, more distant galaxies that are more difficult to observe. In interstellar clouds in the Milky Way, both iron and silicon are generally largely absent from the gas, and thus are thought to be major constituents of the dust.

In the gas toward Sk 155, however, the team discovered clouds with very low iron abundances but surprisingly high silicon abundances. This seems to imply that little, if any, silicon is present in the dust there. If the dust in the more distant galaxies also is deficient in silicon, then the total (gas plus dust) silicon abundance would be smaller than previously thought.

"This could be a breakthrough in actually measuring the different ways in which solid particles can form and develop in space, which ultimately will help us understand star formation," added Donald York, the Horace B. Horton Professor in Astronomy & Astrophysics at Chicago.

The possibility that the dust in the Small Magellanic Cloud is made primarily of iron, with little silicon, might in principle affect the abundance of molecular hydrogen, which is thought to form on the dust grains. Molecular hydrogen, in turn, plays a significant role in star formation. A cloud of gas has to cool to collapse into a star, and molecular hydrogen promotes the cooling process. "One needs grains to produce molecular hydrogen to produce cooling to produce stars," York said.

The team now has taken Hubble Telescope observations of the interstellar matter along the sightlines to two stars in the Small Magellanic Cloud. Their 1995 observations of the star Sk 108 hinted at differences in heavy elemental abundances, but too little dust was present to reveal the clear differences seen toward Sk 155.

Now Welty would like to use the Hubble Telescope to collect data along other Small Magellanic Cloud sightlines to see just how typical these most recent elemental abundance data are for that neighboring galaxy. - By Steven N. Koppes

[Contact: Steve Koppes]






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