London: Astronomers have detected the first noble gas molecules in space in the Crab Nebula, a supernova remnant.
Led by Prof Mike Barlow from University College London`s Department of Physics & Astronomy the team used European Space Agency`s Herschel Space Observatory to observe the Crab Nebula in far infrared light.
Their measurements of regions of cold gas and dust led them to the discovery of the chemical fingerprint of argon hydride ions.
Before the discovery, molecules of this kind have only been studied in laboratories on Earth.
"We were doing a survey of the dust in several bright supernova remnants using Herschel, one of which was the Crab Nebula," Barlow said.
"Discovering argon hydride ions here was unexpected because you don`t expect an atom like argon, a noble gas, to form molecules, and you wouldn`t expect to find them in the harsh environment of a supernova remnant," said Barlow.
In addition to mapping the dust by making far-infrared images of the nebula, the team used Herschel`s SPIRE instrument to make spectroscopic observations.
In these, the infrared light is split up and dispersed according to its wavelength, much like a prism breaks white light down into its respective colours.
When they looked at the data, the team saw some very unusual features which took some time to fully understand.
"Looking at infrared spectra is useful as it gives us the signatures of molecules, in particular their rotational signatures," Barlow said.
"Where you have, for instance, two atoms joined together, they rotate around their shared centre of mass. The speed at which they can spin comes out at very specific, quantised, frequencies, which we can detect in the form of infrared light with our telescope," he said.
Elements can exist in several different versions, or isotopes, which have different numbers of neutrons in their atomic nuclei.
The properties of isotopes are very similar to one another in most respects, but they do differ slightly in mass. Because of this mass difference, the speed of rotation depends on which isotopes are present in a molecule.
The light coming from certain regions of the Crab Nebula showed extremely strong and unexplained peaks in intensity around 618 Gigahertz and 1235 GHz.
Consulting databases of known properties of different molecules, the scientists found that the only possible explanation was that the emission was coming from spinning molecular ions of argon hydride.
Moreover, the only isotope of argon whose hydride could rotate at that rate was argon-36.