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A composite image of the crab nebula made up of the Hubble Space Telescope image (blue) and the Herschel PACS image (red), which shows the location of the dusty filaments. The brightest (white) parts of the image show the locations of the dust and argon hydride molecules newly discovered in this remnant. (Credit: ESA/HERSCHEL/SPIRE and PACS/MESS GTKP supernova remnant team. NASA/ESA/Allison Loll/Jeff Hester)

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Noble gas molecule found in remains of exploded star

Astronomers have discovered a molecule in space that contains a noble gas. Until now, scientists thought such compounds only existed on Earth.

The find was made using an instrument aboard Europe’s Herschel Space Observatory. The molecule, argon hydride, was seen in the crab nebula, the remains of a star that exploded 1,000 years ago

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.

An image of the crab nebula taken with the Herschel PACS instrument showing the location of the dusty filaments. The graph below shows the emission lines which allowed the team to identify and discover the ionised argon hydride molecules for the first time. (Credit: ESA/HERSCHEL/SPIRE and PACS/MESS GTKP supernova remnant team)
An image of the crab nebula taken with the Herschel PACS instrument showing the location of the dusty filaments. The graph below shows the emission lines which allowed the team to identify and discover the ionised argon hydride molecules for the first time. (Credit: ESA/HERSCHEL/SPIRE and PACS/MESS GTKP supernova remnant team)

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“The crab nebula was only formed 1,000 years ago when a massive star exploded,” says Haley Gomez of Cardiff University’s School of Physics and Astronomy. “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.

“Last year, we used the European Space Agency’s Herschel Space Observatory to study the intricate network of gas filaments to show how exploding stars are creating huge amounts of space dust.”

Further measurements of the crab nebula were made using Herschel’s SPIRE 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 wavelengths, or colors, called “emission lines.”

The precise wavelength is dictated 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.

Bizarre discovery

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 Gomez.

“The team had a hard time figuring out what these lines were from, as no one had seen them before.”

Barlow adds: “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 ionized, or energized, 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.

The measurements allowed the team to gauge 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,” says 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 1,000 years ago.”

The details of the discovery are reported in the journal Science.

Source: Cardiff University

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