Date: 01 Sep 2010
Satellite: Herschel
Copyright: ESA. Adapted from L. Decin et al. (2010)

This figure illustrates some of the chemical processes that occur in the envelope surrounding a carbon-rich Asymptotic Giant Branch (AGB) star. Ultraviolet (UV) photons penetrating the clumpy envelope trigger a set of chemical reactions that liberate oxygen, which is locked up in molecules of carbon monoxide (CO) and silicon monoxide (SiO), and result in the production of water vapour (H₂O).

Another molecule produced by UV-induced photochemistry is cyanoacetylene (HC₃N).

The newly proposed mechanism in which UV photons penetrate the clumpy envelope surrounding the carbon star is the only mechanism that can account for the production of water in the inner part of the circumstellar envelope, at a distance from the central star less than 5 times the radius of the star, where the temperature is about 1000 Kelvin or more.

Another mechanism proposed in the past, namely the vaporisation of comets and other icy bodies, can only account for the production of water in the less dense intermediate circumstellar envelope (at distances from the central star up to 100 times the radius of the star, where the temperature drops to about 100 Kelvin) and in the tenuous, outer envelope (at distances from the central star up to 20 000 times the radius of the star and temperatures of the order of 10 Kelvin). Such a mechanism would not explain the recent Herschel detection of water lines corresponding to temperatures up to 1000 Kelvin.