Warm water vapour in the sooty outflow from a luminous carbon star

The detection1 of circumstellar water vapour around the ageing carbon star IRC 110216 challenged the current understanding of chemistry in old stars, because water was predicted² to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star1, grain surface reactions³, and photochemistry in the outer circumstellar envelope⁴. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC 110216 using the Herschel satellite⁵. This includes some high-excitation lines with energies corresponding to ∼1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances⁶ are much higher than hitherto predicted⁷.