An international team of astronomers has accidentally spotted the first space molecules bearing a noble gas, argon. The surprising discovery, in the debris of an exploded star, reveals the element's isotopic composition, confirming long-standing predictions that argon is forged in such doomed stars.



Looking for common molecules

Barlow and his colleagues wanted to observe the Crab Nebula's dust, which radiates its heat at the far-infrared wavelengths that Herschel detects. They also searched the Herschel spectra for lines from common molecules such as carbon monoxide.

The scientists never found those molecules. Instead, they saw two mysterious emission lines – one at a wavelength of 243 microns, the other at 486 microns, exactly twice as long. "That was a giveaway that it was a simple diatomic molecule – two atoms rotating about each other," says Barlow. After failing to find a match with common diatomic molecules, the scientists realized that they had spotted the argon hydride molecular ion, the chemical formula of which is ArH+.

"It was very surprising to us," Barlow says. "Nobody had predicted the molecule. We call the discovery serendipitous to make it sound a bit more scientific, but it was an accident – a lucky discovery." The peculiar molecule probably forms when singly ionized argon – an argon atom with one of its electrons missing – meets molecular hydrogen (H2) and grabs a hydrogen atom.


Lighter argon isotope

Mike Barlow, University College London

But theorists have long predicted that massive stars should manufacture large quantities of a lighter argon isotope, argon-36, which has equal numbers of protons and neutrons. Other astronomers had already detected argon atoms in the Crab Nebula. "But there was no direct proof that it was argon-36," Barlow says, because atomic spectral lines from different argon isotopes have nearly the same wavelengths, making it difficult to distinguish them.

For molecules, however, the task is easy, because molecules containing different argon isotopes emit radiation at noticeably different wavelengths. Therefore, the argon-hydride molecules revealed the element's isotopic composition: it is argon-36, just as the theory predicts.

Cherished beliefs borne out



Massive stars should also produce smaller amounts of another argon isotope, argon-38. If scientists can detect it, they could compare its abundance with that of argon-36. "It's a direct test of nuclear reaction theory in supernovae," says Barlow. He hopes to use ground-based telescopes to search the Crab Nebula for the heavier argon isotope, because the Herschel Space Observatory recently ceased observations.
