A molecule¹ containing a noble gas has been discovered in space by a team including astronomers² from Cardiff University. The find was made using a Cardiff-led instrument aboard Europe's Herschel Space Observatory³. The molecule, argon（氩） hydride（氢化物）, was seen in the Crab⁴ Nebula⁵, the remains⁶ of a star that exploded 1,000 years ago. Before the discovery, molecules⁷ of this kind have only been studied in laboratories on Earth.

 

The noble gases, which include helium（氦）, argon, radon（氡） and krypton（氪）, 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 1000 years ago when a massive star exploded," said Dr Haley Gomez of Cardiff University's School of Physics and Astronomy. "Not only is it very young in astronomical⁸ terms, but also relatively⁹ close, at just 6,500 light years away, providing an excellent way to study what happens in these stellar explosions. Last year, we used the European Space Agency's Herschel Space Observatory to study the intricate network of gas filaments¹⁰ to show how exploding stars are creating huge amounts of space dust."

 

Further measurements of the Crab Nebula were made using Herschel's SPIRE¹¹ instrument. Its development and operation was led by Professor Matt Griffin, from the School of Physics and Astronomy. As molecules spin in space, they emit light of very specific wavelengths¹², or colours, called "emission¹⁴ lines." The precise wavelength¹³ is dictated¹⁵ by the composition and structure of the molecule. Studying the emission lines observed by the SPIRE instrument allows astronomers to study the chemistry of outer space.

 

The team, led by Professor Mike Barlow from University College London, did not set out to make the discovery, but stumbled upon it almost by accident. "We were really concentrating on studying the dust in the filaments with SPIRE, and out pops these two bright emission lines exactly where we see the dust shining," says Dr Gomez. "The team had a hard time figuring out what these lines were from, as no-one had seen them before."

 

Professor Barlow said, "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."

 

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 ionised, or energised, 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 molecular¹⁶ hydrogen, made of two hydrogen atoms. The ionised argon then mixed with the cool gas to provide perfect conditions for noble gas compounds to form.

 

The measurements allowed the team to gauge¹⁷ other properties in argon molecules. "Finding this kind of molecule allowed us to evaluate the type (or isotope) of argon we discovered in the Crab Nebula," said Dr Gomez. "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 1000 years ago."

 

"What a great detective story," added Prof Matt Griffin, from Cardiff University, and lead scientist of the team behind the SPIRE instrument. "Here we see the excellent performance of the Herschel-SPIRE spectrometer, the expertise¹⁸ of the instrument team in producing the highest quality data, and the tenacity¹⁹ and vision of the scientists analysing it, all coming together to make an intriguing²⁰ new discovery."

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