Major galactic mystery solved by CU astronomers

Researchers at the University of Colorado at Boulder have solved a major galactic mystery that may help astronomers in their quest to develop a detailed picture of the chemical evolution of the Milky Way galaxy.

Speaking at the 204th meeting of the American Astronomical Society held May 30 – June 3 in Denver, the researchers reported that the abundance of deuterium, a heavy form of hydrogen, in the Milky Way galaxy today shows a consistent pattern that can be simply explained, lifting a veil of uncertainty that has long plagued astronomers.

Research Professor Jeffrey Linsky of the CU-Boulder astrophysical and planetary sciences department and a fellow at JILA, and Senior Research Associate Brian Wood of JILA, reported the findings. JILA is a joint institute of CU-Boulder and the National Institute for Standards and Technology.

“For astronomers to be able to answer questions such as whether life exists elsewhere, we have to understand the whole picture of the chemical abundances in galaxies, and measuring deuterium is our best test,” Linsky said. “However, much of the deuterium is locked up in dust grains that can’t easily be measured. What we’ve done is come up with a way to measure the total abundance of deuterium in the galaxy.”

Analyzing data obtained by NASA’s Far Ultraviolet Spectroscopic Explorer, or FUSE, and by previous satellites, the researchers found the total ratio of deuterium to hydrogen in gas between stars out to 3,000 light years from the sun is 23 parts per million. That ratio is only slightly smaller than the best estimates of the ratio at the beginning of the universe, which was about 28 parts per million.

The small difference between the primordial ratio and the new value for the Milky Way provides challenges to the current understanding of galactic chemical evolution, Linsky said. It could indicate a much smaller amount of chemical evolution in our galaxy, a much higher value for the assumed infall of near primordial gas to the Milky Way galaxy, some as yet unknown process or a combination of these effects.

“Astronomers are developing a detailed picture of galactic chemical evolution, but the abundance of deuterium, one of the key tracers of this evolution, has been highly uncertain until now,” Linsky said.

Since most of the deuterium present in the universe was created at the time of the Big Bang about 14 billion years ago, the ability to accurately measure it today will allow astronomers to have a better understanding of the chemical evolution of the Milky Way and other galaxies. It is widely believed that over time the abundance of deuterium in the universe has decreased as it is converted to helium and other heavier elements by nuclear reactions in the hot interiors of stars. When stars explode as supernovae or lose matter through winds, this nuclear-processed material — which contains much less deuterium but is rich in heavier elements like oxygen and iron — is blown out into the galaxy, Linsky said.

“The next generations of stars, planets — and eventually human beings — are formed out of this processed material,” Linsky said.

Deuterium’s abundance in gas between the stars is best measured by its characteristic absorption of ultraviolet light from stars that can only be observed by satellites in space. Beginning in 1972 with observations by the Copernicus satellite, measurements of the abundance of deuterium in interstellar gas have presented a confusing picture with a wide range of values for different lines of sight in our galaxy. Previous reports by FUSE researchers of interstellar deuterium abundances along many lines of sight confirm the wide range of deuterium abundances, but until now they have not provided a detailed explanation for this wide range, according to Linsky.

All short lines of sight toward stars within 300 light years of the sun show essentially the same ratio of deuterium to hydrogen of 15 parts per million. This nearby region of the Milky Way galaxy, called the Local Bubble, has a common history of star formation and supernovae events so the interstellar gas is well mixed.

Linsky and Wood believe that the low values of the deuterium-to-hydrogen ratio observed by FUSE toward more distant stars are caused by the depletion of deuterium from the gas phase onto dust grains in those regions of space where there has not been recent supernovae events or nearby hot stars to evaporate the dust and return its deuterium back to the gas phase. The theoretical basis for deuterium depletion onto dust grains was recently described in detail by Professor Bruce Draine of Princeton University.

Since FUSE and other satellites only measure the absorption of deuterium in the gas phase, the total amount of deuterium can be much larger than the gas phase values. Linsky and Wood describe observations that show the most likely ratio, including both gas and dust phases, is about 23 parts per million.

The FUSE satellite was launched in 1999 and is a joint project of NASA, the Canadian Space Agency, the French space agency CNES, the Johns Hopkins University, CU-Boulder and the University of California, Berkeley. The work is supported by a NASA contract through the Johns Hopkins University.