  In 2001, the discovery of circumstellar water vapour around the ageing carbon
star IRC+10216 was announced. This detection challenged the current
understanding of chemistry in old stars, since water vapour was predicted to be
absent in carbon-rich stars. Several explanations for the occurrence of water
vapour were postulated, including the vaporization of icy bodies (comets or
dwarf planets) in orbit around the star, grain surface reactions, and
photochemistry in the outer circumstellar envelope. However, the only water
line detected so far from one carbon-rich evolved star can not discriminate, by
itself, between the different mechanisms proposed. Here we report on the
detection by the Herschel satellite of dozens of water vapour lines in the
far-infrared and sub-millimetre spectrum of IRC+10216, including some
high-excitation lines with energies corresponding to ~1000 K. The emission of
these high-excitation water lines can only be explained if water vapour is
present in the warm inner region of the envelope. A plausible explanation for
the formation of warm water vapour appears to be the penetration of ultraviolet
(UV) photons deep into a clumpy circumstellar envelope. This mechanism triggers
also the formation of other molecules such as ammonia, whose observed
abundances are much higher than hitherto predicted.
