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03 September 2010
The European Space Agency’s Herschel infrared space observatory has discovered that ultraviolet starlight is a key ingredient for making water in the atmosphere of some stars.
According to latest research this is the only explanation for why a dying star is surrounded by a gigantic cloud of hot water vapour.
These new results have been possible through the use of the SPIRE instrument, which was built by an international consortium and led by the University’s School of Physics and Astronomy. The findings have been published in the prestigious scientific journal Nature.
When astronomers discovered an unexpected cloud of water vapour around the old star CW Leonis in 2001, they immediately began searching for the source. Water is known to be present around several types of stars, but CW Leonis is a ‘carbon star’ and therefore thought not to produce water. Initially they suspected the star’s heat must be evaporating comets or even dwarf planets to produce the water.
Using spectrometers, the SPIRE and Photoconductor Array Camera and Spectrometer (PACS) instruments on board Herschel can not only identify molecules such as water vapour, but also its temperature.
They have revealed that the secret ingredient is ultraviolet light as the water vapour is too hot to have come from the destruction of icy celestial bodies and is distributed throughout the stellar wind, including deep down near the surface of the star itself. This suggests that the water is being created by a previously unsuspected chemical process where ultraviolet radiation from interstellar space is breaking up the carbon monoxide and releasing oxygen atoms that can then react with hydrogen to form water molecules.
Professor Matt Griffin, School of Physics and Astronomy and Principal Investigator for Herschel's SPIRE instrument, said: "This is the kind of result that the people who built SPIRE love to see - a real surprise, and one that makes us re-think our ideas about how stars evolve. The discovery of water where it shouldn't really exist shows the power of Herschel to reveal new aspects of the Universe."
Ultraviolet starlight would normally be blocked by the material flowing from the star as its outer layers billow out in a stellar wind. It was already known that the stellar wind is "clumpy" but the Herschel results have shown that some regions around the star must have no wind. These empty regions allow the ultraviolet light to reach the deepest layers of the star’s atmosphere and initiate the chemical reactions that produce the water.
Professor Walter Gear, Dr Pete Hargrave and Dr Haley Gomez also from the University’s School of Physics and Astronomy are part of the international consortium led by Dr Leen Decin of K.U. Leuven University, Belgium.
Dr Gomez said: "The remarkable thing about this result is that it was so out of the blue. We really did not expect to see warm water molecules in the sooty, carbon-rich material around this grown-up star. This finding will have enormous implications for our understanding of the complex molecules and rich chemistry in these atmospheres but it also hints (for the first time) that the solid cosmic dust grains which will inevitably form out of this water-rich gas will be very different from what we previously thought."
The spectrometer within the SPIRE instrument is unique as it allows emission from specific elements and molecules to be observed across the whole of its wavelength range in a single observation. This has never been possible with instruments observing at these wavelengths, and is invaluable for determining which molecules are present, and also for determining temperatures. These results rely on the measurements of emission over a wide range of wavelengths. SPIRE is observing wavelengths several hundred times longer than visible light, some of which have never been observed before for astronomical purposes.
CW Leonis is a red giant star which is only a few times the mass of the Sun but has expanded to hundreds of times its size. Nuclear fusion reactions deep inside the star are converting helium into carbon, much of which has ended up in the outer layers of the star’s atmosphere. It is this abundance of Carbon in the atmospheres of these types of stars that has previously led scientists to believe that water could not exist; with so much carbon all of the oxygen should be locked up in carbon monoxide (CO).
Professor Griffin said: "What this result and others are telling us is that there is water everywhere in the Universe - we are seeing it in comets, planets, all over the place in our galaxy and in other galaxies. With Herschel we are going to be able to understand how it helps shape the development and evolution of stars and planets. Results such as this are very satisfying for those who worked for so long to build the observatory and its instruments."
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