Led by Professor Mike Barlow (UCL Physics & Astronomy) the team used ESA’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 serendipitous discovery of the chemical fingerprint of argon hydride ions, published today in the journal Science. The findings support scientists’ theories of how argon forms in nature.
The Herschel Space Observatory, an ESA space telescope which recently completed its mission, is the biggest space telescope ever to have flown. Herschel’s instruments were designed to detect far-infrared light, which has much longer wavelengths than we can see with our eyes.
“We were doing a survey of the dust in several bright supernova remnants using Herschel, one of which was the Crab Nebula. 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.
Although hot objects like stars glow brightly in visible light, colder objects like the dust in nebulae radiate mainly in the infrared, wavelengths which are blocked by Earth’s atmosphere. Although nebulae can be seen in visible light, this light comes from hot excited gases within them; the cold and dusty component is invisible at optical wavelengths.
“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.”
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.