Playing Hide and Seek With Deuterium

The star AE Aurigae in an area of space containing carbon-rich dust grains, which astronomers think could be hiding deuterium.

T. Rector and B. Wolpa / NOAO / AURA / NSF

For years, astrophysicists have been engaged in a tricky form of space chemistry: trying to figure out how much of a hydrogen variant known as deuterium should have been produced by the big bang. The knowledge is key to understanding how the cosmos has evolved, but new data show reality isn't squaring with predictions.

Deuterium is identical to hydrogen, save for a neutron in its nucleus that makes it twice as heavy. The element floats freely in space until consumed within the nuclear fires of stars to produce helium. Cosmological models have predicted this consumption rate, but recent observations have produced two conundrums: Considerably more deuterium exists than expected, and the stuff does not seem to be spread uniformly across the galaxy.

Now, astrophysicist Jeffrey Linsky of the University of Colorado in Boulder and colleagues think they have resolved the latter question with six years' worth of data on the Milky Way Galaxy collected by NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft. Reporting in the 20 August issue of The Astrophysical Journal, Linsky's team notes that deuterium's spectral signal is weaker within clouds of interstellar dust than in relatively dust-free regions. "Where there are high concentrations of interstellar dust in the galaxy, we see lower concentrations of deuterium gas," Linsky says. Conversely, where there is less dust, there's more gas. That means some deuterium must be binding chemically to interstellar dust grains in the frigid cold of deep space, the team reports, changing in the process from its easily detectable gaseous form to a relatively stealthy solid.

The question remains, however, why there's more deuterium out there than predictions allow. Theoretical models say 33% of the gas created by the big bang and present in the Milky Way should have been consumed by stars. But FUSE findings show a loss of only about 15%. This means either the stars are consuming much less deuterium than expected, or "much more primordial gas has rained down onto the galaxy over its lifetime than had been thought," Linsky says. "In either case, our models of the chemical evolution of the Milky Way will have to be revised significantly."

Astrophysicist Don York of the University of Chicago suggests that the challenge may lie with the limitations of current technology. "This is a very tough problem, and the equipment we have now is not up to answering these sophisticated questions," he says. As far as hydrogen and deuterium are concerned, York says, there needs to be at least a fivefold increase in resolving power.

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