Herschel finds warm water on red giant star
Astronomers using ESA'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 scientists
used data from the SPIRE and PACS instruments, which made it possible not only to detect water vapour by
its "wavelength fingerprint" but also to measure its temperature. The PACS spectrometer has been developed
by a consortium of institutes led by the Max Planck Institute for Extraterrestrial Physics.
CW Leonis is a red giant star in the constellation of Leo, a star at the end of its lifetime that -
although it is only a few times the mass of the Sun - has expanded to hundreds of times its size. It
is the brightest star in the sky when observed in the infrared, but barely detectable at visible wavelengths.
The red giant carbon star CW Leonis as seen by the PACS and SPIRE cameras on board Herschel. The star
itself is too bright to be seen well, but it is releasing material in a violent stellar wind, some of
which is seen in a 'bow shock' to the left of the star in this image. Observations with the PACS and
SPIRE spectrometers have shown that water vapour is being formed deep down near the surface of the star;
a place where it was previously thought to be impossible. This means that the stellar wind must be much
more 'clumpy' than previously thought, with some regions having a much weaker wind than others. This
allows ultraviolet light from interstellar space to reach the deeper, warmer regions and trigger the
creation of water vapour.
Credit: ESA / SPIRE & PACS
The star is classified as a 'carbon-star', as much of the carbon produced by nuclear fusion reactions deep
inside the star has ended up in the outer layers of the star's 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 have now 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.
Warm water vapour in the sooty outflow from a luminous carbon star
L. Decin, M. Agúndez, M. J. Barlow, et al.
Nature 467, 64–67 (2010)
Institute for Astronomy of the Katholieke Universiteit Leuven, Belgien