A molecule containing a noble gas has been discovered in space by a team including astronomers from Cardiff University.
The find was made using a Cardiff-led 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. Before the discovery, molecules of this kind have only been studied in laboratories on Earth.
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 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 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 colours, 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.
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 molecular 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 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," 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 expertise of the instrument team in producing the highest quality data, and the tenacity and vision of the scientists analysing it, all coming together to make an intriguing new discovery."
Notes for editors
The results described in this article are reported in "Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula", by M. J. Barlow et al., published in the Dec 13th issue of Science, Volume 342, Issue 6164, DOI: 10.1126/science.1243582
The argon isotope found in the Crab Nebula is different from the one that dominates in Earth's atmosphere, 40Ar, which derives from the decay of a radioactive isotope of potassium (40K) present in our planet's rocks. At almost one per cent, argon is the third most abundant gas in the atmosphere of Earth after nitrogen and oxygen, and was discovered at the end of the 19th century.
The element argon was first discovered by William Ramsay in the 1890's, in his laboratory at University College London.
The study is based on data collected with the Spectral and Photometric Imaging Receiver (SPIRE) on board ESA's Herschel Space Observatory. The team of astronomers detected two emission lines corresponding to the first two rotational transitions of argon hydride (ArH+) at frequencies of 617.5 GHz and 1234.6 GHz, respectively. To identify the lines, they made use of two extensive databases of molecular lines: the Cologne Database for Molecular Spectroscopy (CDMS) and the Madrid Molecular Spectroscopy Excitation (MADEX) code.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
The SPIRE instrument contains an imaging photometer (camera) and an imaging spectrometer. The camera operates in three wavelength bands centred on 250, 350 and 500 μm, and so can make images of the sky simultaneously in three sub-millimetre colours; the spectrometer covers the wavelength range between 194 and 671 ìm. SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC, UKSA (UK); and NASA (USA).
Herschel was launched on 14 May 2009 and completed science observations on 29 April 2013.
Prof Matt Griffin (PI SPIRE)
School of Physics and Astronomy
Email: matt.griffin [@] astro.cf.ac.uk
Phone: +44 (0)2920 874 203
Dr Haley Gomez (co-author)
School of Physics and Astronomy
Email: haley.gomez [@] astro.cf.ac.uk
Phone: +44 (0)2920 874058
Dr Chris North (Herschel Outreach Officer)
School of Physics and Astronomy
Email: chris.north [@] astro.cf.ac.uk
Phone: +44 (0)2920 870537
Prof Michael J. Barlow (lead author)
Department of Physics & Astronomy
University College London
Phone: +44 (0)20 7679 7160
Mobile: +44 (0)77 5894 5482