By Balasubramanyam Seshan: Subscribe to Balasubramanyam's RSS feed
September 3, 2010 12:15 PM EDT
A gigantic cloud of hot water vapor surrounding a dying star can be explained by the interaction of ultraviolet starlight breaking up molecules such as carbon monoxide and silicon monoxide, releasing oxygen atoms that then attach themselves to hydrogen molecules forming water, according to the European Space Agency’s Herschel infrared space observatory.
When astronomers discovered an unexpected cloud of water vapors around the old star IRC+10216 in 2001, they immediately began searching for the source. Stars like IRC+10216 are known as carbon stars and are thought not to make much water.
Initially the astronomers suspected the star’s heat must be evaporating comets or even dwarf planets to produce the water. Now, Herschel’s PACS and SPIRE instruments have revealed that the secret ingredient is ultraviolet light, as the water is too hot to have come from the destruction of icy celestial bodies.
The superb sensitivity of Herschel’s instruments has revealed that the water around IRC+10216 varies in temperature from about –200 degree centigrade to +800 degree centigrade, which indicates that it is being formed much closer to the star than comets can stably exist.
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IRC+10216 is a red giant star, hundreds of times the Sun’s size, although only a few times its mass. If it replaced the Sun in the Solar System, it would extend beyond the orbit of Mars.
The star is 500 light years away and while it is barely detectable at visible wavelengths, even in the largest telescopes, it is the brightest star in the sky at some infrared wavelengths.It is surrounded by a huge envelope of dust that absorbs almost all its visible radiation and re-emits it as infrared light. It is in the envelope that the water vapor has been found. Observations had already revealed the clumpy structure in the dusty envelope around IRC+10216.
The Herschel water detection made the astronomers realize that ultraviolet light from surrounding stars can reach deep into the envelope between the clumps and break up molecules such as carbon monoxide and silicon monoxide, releasing oxygen atoms. The oxygen atoms then attach themselves to hydrogen molecules, forming water.
“This is the only mechanism that explains the full range of the water’s temperature. The closer to the star the water is formed, the hotter it will be. We are very hopeful that Herschel will find the same situations around those stars too,” said Leen Decin, Katholieke Universiteit Leuven, Belgium.
Decin and her colleagues now plan to extend the observations to other carbon stars. On Earth, carbon compounds and water are the key ingredients for life. Now, the astronomers know that both carbon compounds and water can be made around IRC +10216, and the secret ingredient for water is ultraviolet light from surrounding stars.
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