Astronomie - Erste Edelgas-Moleküle im Weltraum


Argon was forged in the doomed star that became the famous Crab Nebula: image taken by the Hubble Space Telescope. (Courtesy: NASA, ESA, J Hester and A Loll (Arizona State University))


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.
Once called inert gases, the elements in the final column of the periodic table have closed outer shells of electrons that normally prevent them from exchanging electrons with other atoms to form molecules. In 1962, however, chemists discovered molecules containing xenon and now call these elements noble gases instead. But no-one had ever seen a molecule in space harbouring a noble gas, even though one such gas – helium – is the universe's second most abundant element.
Mike Barlow, an astronomer at University College London, and his colleagues were using the Herschel Space Observatory to study supernova remnants, including the well known Crab Nebula. It resulted when a massive star 6500 light-years from Earth in the constellation Taurus ran out of fuel, sparking a brilliant explosion that our ancestors witnessed in 1054.
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.
Argon (atomic number 18) is the eleventh most abundant element in the universe and the third most common gas in the atmospheres of Venus, Earth and Mars. The element makes up 0.93% of the air that we breathe. Most terrestrial argon is argon-40, which comes from the decay of radioactive potassium-40 in rocks.
Lighter argon isotope
We call the discovery serendipitous to make it sound a bit more scientific, but it was an accident – a lucky discovery 
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
"It's nice to see cherished beliefs borne out," says astronomer Stan Woosley, a supernova expert at the University of California at Santa Cruz, who was not involved with the discovery. Early in its life, a massive star shines by converting hydrogen into helium, as the Sun does. Then the star begins burning the helium into carbon and oxygen, which eventually forge still heavier elements. Argon arises during the oxygen-burning stage, in which one oxygen-16 nucleus hits another, creating sulphur-32. The sulphur nucleus is in an excited state and usually emits a helium-4 particle, thereby becoming silicon-28. The helium-4 particles strike the silicon-28 and sulphur-32 nuclei to make argon-36 and also calcium-40.
Woosley says that the timing of argon's creation depended on the star itself, which astronomers think was born eight to 16 times as massive as the Sun. If the lesser figure is correct, he says the element originated primarily in the supernova. If instead the star was born with the larger mass, it created most of its argon before the explosion, during the last few months of its life.
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.
Barlow and his colleagues are publishing their discovery online today in Science.
Quelle: Physics World

Astronomers discover first noble gas molecules in space

Noble gas molecules have been detected in space for the first time in the Crab Nebula, a supernova remnant, by astronomers at UCL.
Led by Professor Mike Barlow (UCL Department of Physics & Astronomy) the team used ESA's Herschel Space Observatory to observe the Crab Nebula in far infrared light.
Their measurements of regions of cold gas and dust led them to the serendipitous discovery of the chemical fingerprint of argon hydride ions, published today in the journal Science.
The findings support scientists' theories of how argon forms in nature.
The Herschel Space Observatory, an ESA space telescope which recently completed its mission, is the biggest space telescope ever to have flown.
Herschel's instruments were designed to detect far-infrared light, which has much longer wavelengths than we can see with our eyes.
"We were doing a survey of the dust in several bright supernova remnants using Herschel, one of which was the Crab Nebula. Discovering argon hydride ions here was unexpected because you don't expect an atom like argon, a noble gas, to form molecules, and you wouldn't expect to find them in the harsh environment of a supernova remnant," said Barlow.
Although hot objects like stars glow brightly in visible light, colder objects like the dust in nebulae radiate mainly in the infrared, wavelengths which are blocked by Earth's atmosphere.
Although nebulae can be seen in visible light, this light comes from hot excited gases within them; the cold and dusty component is invisible at optical wavelengths.
In addition to mapping the dust by making far-infrared images of the nebula, the team used Herschel's SPIRE instrument to make spectroscopic observations. In these, the infrared light is split up and dispersed according to its wavelength, much like a prism breaks white light down into its respective colours. When they looked at the data, the team saw some very unusual features which took some time to fully understand.
"Looking at infrared spectra is useful as it gives us the signatures of molecules, in particular their rotational signatures," Barlow said. "Where you have, for instance, two atoms joined together, they rotate around their shared centre of mass. The speed at which they can spin comes out at very specific, quantised, frequencies, which we can detect in the form of infrared light with our telescope."
Elements can exist in several different versions, or isotopes, which have different numbers of neutrons in their atomic nuclei. The properties of isotopes are very similar to one another in most respects, but they do differ slightly in mass. Because of this mass difference, the speed of rotation depends on which isotopes are present in a molecule.
The light coming from certain regions of the Crab Nebula showed extremely strong and unexplained peaks in intensity around 618 Gigahertz and 1235 GHz. Consulting databases of known properties of different molecules, the scientists 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.
In this case, energy from the neutron star at the heart of the nebula appears to have ionised the argon, which then joined with molecules of hydrogen to form the molecular ion ArH+.
Professor Bruce Swinyard (UCL Department of Physics & Astronomy and Rutherford Appleton Laboratory), a member of the team, added: "Our discovery was unexpected in another way – because normally when you find a new molecule in space, its signature is weak and you have to work hard to find it. In this case it just jumped out of our spectra."
The discovery of argon-36 in the Crab Nebula, as well as being the first detection of its kind, helps support scientists' theories of how argon forms in nature. Calculations of what elements are churned out by a supernova predict a lot of argon-36 and no argon-40 – exactly what the team observed in the Crab Nebula. On Earth, however, argon-40 is the dominant isotope as it is released by the radioactive decay of potassium in rocks.
This first discovery of an argon molecule in space continues a long tradition of noble gas research at UCL. Argon, along with the other noble gases, was discovered at UCL by William Ramsay at the end of the 19th century.
Quelle: EurekAlert
This image shows a composite view of the Crab nebula, an iconic supernova remnant in our Milky Way galaxy, as viewed by the Herschel Space Observatory and the Hubble Space Telescope. Image credit: ESA/Herschel/PACS/MESS Key Programme Supernova Remnant Team; NASA, ESA and Allison Loll/Jeff Hester (Arizona State University)
Crab Nebula, as Seen by Herschel and Hubble
This image shows a composite view of the Crab nebula, an iconic supernova remnant in our Milky Way galaxy, as viewed by the Herschel Space Observatory and the Hubble Space Telescope. Herschel is a European Space Agency (ESA) mission with important NASA contributions, and Hubble is a NASA mission with important ESA contributions.
A wispy and filamentary cloud of gas and dust, the Crab nebula is the remnant of a supernova explosion that was observed by Chinese astronomers in the year 1054.
The image combines Hubble's view of the nebula at visible wavelengths, obtained using three different filters sensitive to the emission from oxygen and sulphur ions and is shown here in blue. Herschel's far-infrared image reveals the emission from dust in the nebula and is shown here in red.
While studying the dust content of the Crab nebula with Herschel, a team of astronomers have detected emission lines from argon hydride, a molecular ion containing the noble gas argon. This is the first detection of a noble-gas based compound in space.
The Herschel image is based on data taken with the Photoconductor Array Camera and Spectrometer (PACS) instrument at a wavelength of 70 microns; the Hubble image is based on archival data from the Wide Field and Planetary Camera 2 (WFPC2).
Herschel is a European Space Agency cornerstone mission, with science instruments provided by consortia of European institutes and with important participation by NASA. NASA's Herschel Project Office is based at NASA's Jet Propulsion Laboratory, Pasadena, Calif. JPL contributed mission-enabling technology for two of Herschel's three science instruments. The NASA Herschel Science Center, part of the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena, supports the United States astronomical community. Caltech manages JPL for NASA.

Chemical Surprise Found in Crab Nebula

Astronomers have discovered a rare chemical pairing in the remains of an exploded star, called the Crab nebula. A gas thought to be a loner has made a "friend," linking up with a chemical partner to form a molecule. The discovery, made with the Herschel space observatory, a European Space Agency mission with important NASA contributions, will help scientists better understand supernovas, the violent deaths of massive stars. 
The unexpected find involves a noble gas called Argon, named for its chemical aloofness after the Greek word for "inactive." Noble gases, which also include helium and neon among others, rarely engage in chemical reactions. They prefer to go it alone. 
A new study, led by Michael Barlow from University College London, United Kingdom, and based on spectral data from Herschel, has found the first evidence of such a noble gas-based compound in space, a molecule called argon hydride. The results are published in the journal Science. 
"The strange thing is that it is the harsh conditions in a supernova remnant that seem to be responsible for some of the argon finding a partner with hydrogen," said Paul Goldsmith of NASA's Jet Propulsion Laboratory, Pasadena, Calif. 
"This is not only the first detection of a noble-gas based molecule in space, but also a new perspective on the Crab nebula. Herschel has directly measured the argon isotope we expect to be produced via explosive nucleosynthesis in a core-collapse supernova, refining our understanding of the origin of this supernova remnant," concludes Göran Pilbratt, Herschel project scientist at the European Space Agency. 
Quelle: NASA
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