Astronomers find hot water vapour around a carbon star

Using the European Herschel space telescope, astronomers have found a hot water vapour formed deep into the atmosphere of a red giant pulsating carbon star, CW Leonis.

An international team including astronomers at Observatoire de Paris, Institut de Radio Astronomie Millimetrique and Observatoire de Grenoble in France observed the hot water vapour in such a rare location that was previously thought to be impossible.
 
This should help to understand how this type of evolved star produces and expels key ingredients for all known forms of life.

The major building blocks of life on Earth are water and carbon-based molecules, synthesized in large quantities by stars like the Sun at the end of their lives. When they age, these stars become red giants and puff out their atmospheres.

These have previously been seen to contain either water or carbon-based molecules, and it was thought that these two types of species could not co-exist.

However, new results have overturned this longstanding concept by detecting abundant hot water vapour in the atmosphere of a very carbon-rich red giant star.

CW Leo is a red giant pulsating star in the constellation of Leo. With a mass roughly similar to that of the Sun, it has expanded to hundreds of times the size of our own star - if placed in the Solar System it would extend beyond the orbit of Mars.

Barely detectable in the visible, even by the largest telescopes, it is the brightest star in the sky observed in the infrared, at wavelengths ten times longer than those seen by human eyes.

This suggests that huge quantities of dust particles have condensed around the star. They absorb almost all of its visible radiation and re-emit it in the infrared.

CW Leonis will soon end its life by becoming a hot white dwarf star surrounded by a diffuse planetary nebula -- an extended cloud of gas and dust made up of material presently in its atmosphere and expelled into space afterwards.

However, now, Herschel has detected the definitive signature of water at many more wavelengths. Vapour appears at temperatures of up to 1000 degrees, implying that it is distributed throughout and deep down into the wind.

The model of the stellar wind interacting with a distant icy comet cloud must now be replaced by another one in which water vapor is created by previously unsuspected chemical reactions triggered with help from interstellar ultraviolet radiation. Ultraviolet light breaks up the carbon monoxide, releasing oxygen atoms that can then react with hydrogen to form water molecules.

The only possible source of the ultraviolet light is interstellar space. It was already known that the stellar wind is 'clumpy', and the Herschel results have shown that some regions around the star must be almost empty. These allow the ultraviolet light to reach the deepest layers of the atmosphere and initiate chemical reactions to produce water.

The results have been published in the 2 September 2010 issue of Nature magazine.