A molecule containing the noble gas argon has been discovered in the beautiful Crab Nebula, the remains of a star that exploded 1,000 years ago. Before the discovery, molecules of this kind have only been studied in laboratories on Earth.
The Crab Nebula is a supernova remnant. Observers in China and Japan recorded the supernova nearly 1,000 years ago, in 1054. Image credit: NASA / ESA / J. Hester and A. Loll, Arizona State University.
The noble gases, which include helium, argon, radon and krypton, usually do not react easily with other chemical elements, and are often found on their own. In the right circumstances, however, they can form molecules with other elements. Such chemical compounds have only ever been studied in laboratories on Earth, leading astronomers to assume the right conditions simply do not occur in space.
“The Crab Nebula was only formed 1,000 years ago when a massive star exploded. Not only is it very young in astronomical terms, but also relatively close, at just 6,500 light years away, providing an excellent way to study what happens in these stellar explosions,” said Dr Haley Gomez from Cardiff University, a co-author of the study published in the journal Science.
As molecules spin in space, they emit light of very specific wavelengths, called emission lines. The precise wavelength is dictated by the composition and structure of the molecule.
The light coming from certain regions of the Crab Nebula showed extremely strong and unexplained peaks in intensity around 618 Gigahertz and 1,235 GHz. Consulting databases of known properties of different molecules, Dr Gomez and his colleagues 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.
“At first, the discovery of argon seemed bizarre. With hot gas still expanding at high speeds after the explosion, a supernova remnant is a harsh, hot and hostile environment, and one of the places where we least expected to find a noble-gas based molecule,” said lead author Prof Mike Barlow from the University College London.
It now seems the Crab Nebula provides exactly the right conditions to form such molecules. The argon was produced in the initial stellar explosion, and then ionized with electrons stripped from the atoms in resulting intense radiation as shockwaves. These shockwaves led to the formation of the network of cool filaments containing cold molecular hydrogen, made of two hydrogen atoms. The ionized argon then mixed with the cool gas to provide perfect conditions for noble gas compounds to form.
“Finding this kind of molecule allowed us to evaluate the type of argon we discovered in the Crab Nebula,” Dr Gomez said.
“We now know that it is different from argon we see in rocks on the Earth. Future measurements will allow us to probe what exactly took place in the explosion 1,000 years ago.”
______
Barlow MJ et al. 2013. Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula. Science, vol. 342, no. 6164, pp. 1343-1345; doi: 10.1126/science.1243582
