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Heavy elements found in interstellar gas.

Astronomers for the first time have detected arsenic and selenium in interstellar gas, they report in the Oct. 10 ASTROPHYSICAL JOURNAL LETTERS. Since completing their newly published study, the researchers also have found thallium and lead, the two heaviest elements ever identified in the gas between stars, says team member Jason A. Cardelli of the University of Wisconsin-Madison.

The findings, as well as data on four other heavy elements, stem from Hubble Space Telescope spectra of seven stars. Measuring the abundance of heavy elements in many more regions of space -- a goal that a repaired Hubble could more easily accomplish -- may offer several insights, Cardelli says. These include a better understanding of the composition and origin of interstellar dust and the link between stellar evolution and interstellar gas. He notes that aging stars generate heavy elements and ultimately dump them into space, where newly forming stars incorporate them.

George Wallerstein of the University of Washington in Seattle calls the new work "just the beginning" of a comprehensive study of the interstellar medium.

The findings bring to eight the number of elements heavier than zinc that scientists have detected in interstellar gas. While the familiar process of nuclear fusion at the core of massive stars generates the lighter atomic species, elements heavier than zinc typically form by a different mechanism, neutron capture. In this process, the nuclei of lighter elements absorb neutrons and undergo radioactive decay, transforming into heavier atomic species.

Supernova explosions produce a flood of neutrons, setting the stage for "fast" neutron capture, in which atoms absorb neutrons more rapidly than they undergo radioactive decay. The smaller supply of neutrons found in the helium shell of bloated, elderly stars known as asymptotic giant branch stars prompts "slow" capture, in which atoms absorb neutrons at a slower rate.

Some heavy elements stem primarily from last capture, others mostly from slow capture. Thus, their relative abundance in interstellar gas may serve to mark different types of stellar activity, Cardelli says. But he adds that the variety of activity in any one region complicates interpretation of the measurements.

Cardelli and his colleagues identified the elements by analyzing ultraviolet light from two nearby stars with Hubble's Goddard High-Resolution Spectrograph. As the light passes through the tenuous fog of interstellar gas, specific wavelengths get absorbed by particular elements, creating tiny gaps in the observed spectra.

The low abundance of heavy elements produces faint spectra that only the Hubble instrument could discern, Cardelli notes. He and his co-workers previously had used Hubble to discover gallium, germanium, and krypton in interstellar gas. And in the July 10 ASTROPHYSICAL JOURNAL, Lewis M. Hobbs of the University of Chicago and his colleagues reported identifying tin, until now the heaviest element found between stars.
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Title Annotation:arsenic, selenium, thallium, lead
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Oct 16, 1993
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