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

TL;DR

The paper reports the detection of warm water vapour in the inner envelope of the carbon star IRC+10216 using Herschel, suggesting UV photons penetrate the clumpy envelope and trigger complex chemistry.

Contribution

It provides the first detailed spectral analysis of water vapour in a carbon star, supporting UV-driven chemistry in a previously unexpected environment.

Findings

Detection of dozens of water lines, including high-excitation lines.

Water vapour exists in the warm inner envelope of IRC+10216.

UV photons penetrate the clumpy envelope, enabling complex molecule formation.

Abstract

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.