Astronomers have discovered molecules containing fluorine
near the center of the Milky Way galaxy, giving scientists a new
element with which to study the cosmic chemistry leading to the
formation and destruction of molecules.
The fluorine was detected in hydrogen fluoride gas inside a giant interstellar cloud about 20,000 light-years from Earth. Hydrogen fluoride can now be added to the more than 100 different kinds of molecules that have been detected in interstellar space during the past 30 years.
"There is a whole field, called astro-chemistry, where you are trying to understand where these molecules come from and what the presence of particular molecules tells us about the gas in which they've been observed," said Johns Hopkins astrophysicist David Neufeld, who led a team of scientists making the discovery.
"If you are in this field, there are flavors of chemistry," he said. "There are molecules containing oxygen, and molecules containing carbon, and molecules containing silicon, and molecules containing sulfur. And people are excited to work out the chemistry that's going on for each of these cases. This is a whole new element, a whole new chemistry, if you like, to think about."
The team of astronomers from the United States and Germany used the European Space Agency's Infrared Space Observatory satellite to detect the hydrogen fluoride. They made observations in March with the Long Wavelength Spectrometer, one of four instruments on the satellite, discovering trace amounts of hydrogen fluoride gas in the near vacuum of interstellar space.
The findings will be reported in an article that is to appear in October in the Astrophysical Journal Letters.
In concentrated liquid form, hydrogen fluoride--or hydrofluoric acid as it is known when dissolved in water--is familiar to laboratory chemists as an extremely dangerous and corrosive acid that dissolves glass and severely burns human tissue.
The satellite's sensitive spectrometers enabled the astronomers to detect hydrogen fluoride in space even though its concentration was less than one part per billion. Instruments capable of seeing the far-infrared portion of the spectrum made the discovery possible because the molecules emit radiation in that range.
"Because the Earth's atmosphere is completely opaque to far-infrared radiation, the observations that we carried out are possible only from space," said Neufeld, a professor in the Department of Physics and Astronomy. "The ISO satellite has opened up an exciting new window on the universe by allowing us to observe at far-infrared wavelengths."
The gas cloud is located in the southern constellation Sagittarius. Known to astronomers as Sagittarius B2, the cloud is composed mostly of hydrogen molecules. It is an environment of extremes, with temperatures less than minus 370 degrees Fahrenheit, and pressures more than one hundred trillion times smaller than the atmospheric pressure on Earth.
The hydrogen fluoride finding is especially interesting because of the unusual way that the molecule likely formed in space. Chemists describe fluorine as the most electronegative element, meaning it can attract and hold onto electrons better than any other element. When fluorine is exposed to hydrogen, it attracts hydrogen's single electron, bonding the hydrogen strongly and forming hydrogen fluoride. Most other molecules in space are made in a more roundabout way that involves cosmic rays ionizing--removing electrons--from molecules or atoms in the gas.
"This is a rather exotic butterfly that we've collected," Neufeld said. "The discovery gives us the opportunity to study the chemistry of fluoride molecules in the frigid conditions that characterize the near vacuum of interstellar space. One of the key questions is how these molecules were formed. Our analysis suggests that the hydrogen fluoride we detected was produced by direct chemical reactions between fluorine atoms and hydrogen molecules. Unlike most atoms, fluorine atoms are extremely reactive and attack the relatively inert hydrogen molecules that are the principal constituent of the interstellar gas. The result is hydrogen fluoride."
The next logical step in the research will be to examine the chemistry of fluorine in other regions of space.
"We have only seen it in one place so far," Neufeld said.
The other members of the team that made the hydrogen fluoride discovery are Professors Jonas Zmuidzinas and Thomas Phillips of the California Institute of Technology, and Peter Schilke of the Max-Planck Institute for Radio Astronomy in Bonn, Germany. The satellite was built and launched by the European Space Agency and operated by ESA with the participation of the National Aeronautics and Space Administration. NASA supported the work of Neufeld, Zmuidzinas and Phillips.
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