This form of hydrogen, called deuterium, was created a few minutes after the Big Bang, but has been slowly destroyed as it is burned in stars and converted to heavier elements. In fact, it now turns out, that destruction has been occurring even more slowly than previously thought.
Published in the Aug. 20 issue of the Astrophysical Journal, the FUSE team's new large deuterium survey solves a 35-year-old mystery concerning deuterium's uneven distribution in the Milky Way galaxy even as it poses new questions about how stars and galaxies are made.
"For more than three decades, we have struggled to understand and explain the widely varying levels of deuterium," said Warren Moos, principal investigator of NASA's FUSE mission and a professor in the Henry A. Rowland Department of Physics and Astronomy at the Krieger School of Arts and Sciences at Johns Hopkins. "Though the answer we have found may be unsettling to some, it represents a major step forward in our understanding of chemical evolution."
In space, deuterium -- a form of hydrogen with not only a proton but also a neutron in its nucleus -- produces a telltale spectral fingerprint in the ultraviolet energy range where FUSE conducts observations. That fingerprint can be measured to determine the quantity of deuterium in various places in our galaxy.
Hundreds of hours of observations toward dozens of stars have been scheduled by the JHU FUSE operations crew over the last six years, making this new result possible.
"FUSE was built to attack the deuterium problem," according to William P. Blair, FUSE's chief of observatory operations and physics and astronomy research professor at Johns Hopkins. "It is very gratifying to see this long-anticipated result, and it will surely be a legacy of the FUSE mission."
The story starts in the 1970s, when NASA's Copernicus satellite found the first fragmentary evidence that the Milky Way's deuterium distribution was patchy. That was perplexing, Blair said, because astronomers thought deuterium should be as evenly mixed as other elements in space.
FUSE's sensitivity "has allowed many more deuterium measurements, and for stars at greater distances from the sun," Blair said. Those numerous, distant observations were crucial, he said, for verifying that what Copernicus suggested was true: There does appear to be more deuterium in some other parts of the Milky Way than there is close to our sun.
Additionally, the pattern of deuterium variations FUSE found strongly supports a recent theory that predicts how heavy hydrogen might behave in interstellar space, Blair said.
In 2003, Princeton University's Bruce Draine, a co-author on the new paper, developed computer models that showed how deuterium might bind more readily than light hydrogen to interstellar dust grains, changing from an easily detectable gaseous form to an unobservable solid.
Draine's models suggested that in places like the neighborhood of the sun, relatively undisturbed for eons, a significant amount of deuterium may have disappeared from view in this way. In other areas, disturbed by supernova blasts or nearby hot stars, dust grains would have vaporized, releasing deuterium atoms back into a detectable gaseous form.
In fact, FUSE found deuterium levels of about 15 parts per million measured in our neighborhood and even lower -- as low as 5 parts per million -- elsewhere. But FUSE also found concentrations as high as 23 parts per million in regions where supernovae or hot stars have occurred. The low levels found near here, the FUSE researchers conclude, indicate only that much of the deuterium in this neighborhood is in undetectable solid form, not that it doesn't exist.
"The peak galactic detection levels are likely close to the real total deuterium abundance in the Milky Way, with the rest of it in hiding, not destroyed," said George Sonneborn of NASA Goddard Space Flight Center in Greenbelt, Md., co-author and mission project scientist.
If that's so, scientists have been wrong to believe up to now that at least a third of what was thought to be the original 27 parts per million of local deuterium has been destroyed since the Big Bang. In fact, the current level is only about 15 percent below that original level.
So, the FUSE findings imply either that significantly less deuterium is being converted to helium and heavier elements in stars or that much more deuterium has rained down onto our galaxy over its lifetime than had been previously been thought.
In either case, our models of the chemical evolution of the Milky Way galaxy will have to be revised significantly to explain this new result, the team said.
Lisa DeNike | EurekAlert!
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