Warm water vapour in the sooty outflow from a luminous carbon star
A team of astronomers, using the European Space Agency's Herschel Space Observatory have
observed water vapour being formed somewhere it was previously thought to be impossible:
in the atmosphere of a red giant carbon star. The team, lead by KU Leuven astronomer
Leen Decin, have published their results in the september 2nd issue of the journal Nature.
The major building blocks of life on Earth are water and carbon-based molecules, both of
which are synthesised in large quantities by stars like the Sun as they reach the end of their
lives. When they age, these stars become red giants and puff out their atmospheres. These
atmospheres have previously been seen to contain either water molecules or carbon-based
molecules, and it was thought that these two types of molecules could not co-exist. New
results from the PACS and SPIRE spectrometers onboard the Herschel Space Observatory,
launched in May 2009, have overturned this longstanding concept by detecting abundant
water vapour in the atmosphere of a very carbon-rich red giant star.
CW Leonis is a red giant star in the constellation of Leo, and although it is only a few times
the mass of the Sun, it has expanded to hundreds of times its size - if it replaced the Sun in
our Solar System it would extend beyond the orbit of Mars. While it is barely detectable at
visible wavelengths, even by the largest telescopes, it is the brightest star in the sky when
observed in the infrared, at wavelengths ten times longer than those seen by the human
eye. Its faintness in visible light and its extreme brightness in the infrared are due to the
huge quantities of dust particles that have condensed around it, absorbing almost all its
visible radiation but re-emitting it in the infrared.
The star is classified as a 'carbon-star', and at a distance of around 500 light years is the
closest such star to Earth. 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 currently emitting ten thousand times as much energy as the Sun, and the star's outer
layers are currently billowing out in a 'stellar wind' - the equivalent of our Sun's solar wind but
releasing a billion times more matter every hour. This wind is rich in many different carbonbased
molecules and dust particles which have been detected by astronomers. CW Leonis
will soon end its life by becoming a white dwarf surrounded by a planetary nebula - a cloud
of gas and dust made up of material currently in its atmosphere.
With so much carbon in its atmosphere almost all of the oxygen should be locked up in
carbon monoxide (CO), meaning that there should be no water (H2O) present. However, in
2001 the Submillimeter Wave Astronomy Satellite (SWAS) detected emission from CW
Leonis at a particular wavelength which shows the existence of water vapour. A possible
origin proposed was that the stellar wind was releasing water molecules from a cloud of icy
comets around the star.
However, Herschel has detected the definitive signature of water at many more
wavelengths, and it is now possible to establish the temperature of the water vapour. It is
emitting at temperatures up to 1000 degrees, implying that the water vapour is distributed
throughout the stellar wind, including deep down near the surface of the star itself. The
model of the stellar wind interacting with a distant icy comet cloud must now be replaced by
one in which the water vapour is being created by previously unsuspected chemical
processes. These processes include reactions which are triggered by ultraviolet radiation.
The 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, but this would normally
be blocked by the material flowing from star. It was already known that the stellar wind is
'clumpy', and the Herschel results, published in the scientific journal Nature, have shown that
some regions around the star must be almost empty. 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.
Harsh ultraviolet radiation from our own Sun is absorbed by the Earth's atmosphere,
protecting life on the surface from harm. However, on the primitive Earth this protective
atmosphere was not present, and the ultraviolet light may have played a crucial role in
triggering prebiotic processes that ultimately created the molecular building blocks of life.
The new Herschel results imply that analogous processes operate around the red giant stars
that continue to supply material for new generations of stars and planets in galaxies like our
own Milky Way.
Publication:
L. Decin, M. Agundez, M. J. Barlow, F. Daniel, J. Cernicharo, R. Lombaert, E. De Beck,
P. Royer, B. Vandenbussche, R. Wesson, E. T. Polehampton, J. A. D. L. Blommaert,
W. De Meester, K. Exter, H. Feuchtgruber, W. K. Gear, H. L. Gomez,
M. A. T. Groenewegen, M. Guélin, P. C. Hargrave, R. Huygen, P. Imhof,
R. J. Ivison, C. Jean, C. Kahane et al.
Warm water vapour in the sooty outflow from a luminous carbon star.
Nature 467, p. 64-67 (02 September 2010).
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