Major galactic chemical mystery solved
Speaking at the 204th meeting of the American Astronomical Society held in Denver, researchers of the University of Colorado at Boulder 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. The researchers have solved a major galactic mystery that may help astronomers to develop a detailed picture of the chemical evolution of the Milky Way galaxy.
Research Professor Jeffrey Linsky of the CU-Boulder astrophysical and planetary sciences department and a Senior Research Associate Brian Wood reported the findings. ``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.''
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 by analysing data obtained by NASA's Far Ultraviolet Spectroscopic Explorer, or FUSE, and by previous satellites. That ratio is slightly smaller than the best estimates of the ratio at the beginning of the universe, which was about 28 parts per million. The difference between the new value for the Milky Way and the primordial ratio provides challenges to the current understanding of galactic chemical evolution, Linsky said.
It could indicate a smaller amount of chemical evolution in our galaxy, a higher value for the assumed in-fall 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 abundance of deuterium, has been uncertain until now,'' Linsky said. The ability to measure it today will allow astronomers to have a better understanding of the chemical evolution of the Milky Way and other galaxies since most of the deuterium present in the universe was created at the time of the Big Bang.
It is 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 measured by its characteristic absorption of UV light from stars that can only be observed by satellites in space. 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, called Local Bubble, has a common history of star formation so the interstellar gas is well mixed. Linsky and Wood believe that the low values of the deuterium-to-hydrogen ratio observed by FUSE towards distant stars are caused by depletion of deuterium from the gas phase onto dust grains in those regions of space where there have not been recent supernovae events or nearby hot stars to evaporate the dust and return its deuterium back to the gas phase. — Our Bureau
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