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Stars on the asymptotic giant branch (AGB) lose substantial amounts ofmatter, to the extent that they are important for the chemical evolution of,and dust production in, the universe. The mass loss is believed to increasegradually with age on the AGB, but it may also occur in the form of bursts,possibly related to the thermal pulsing phenomenon. Detached, geometricallythin, CO shells around carbon stars are good signposts of brief and intensemass ejection. We aim to put further constraints on the physical properties ofdetached CO shells around AGB stars. The photodissociation of CO and othercarbon-bearing species in the shells leads to the possibility of detectinglines from neutral carbon. We have therefore searched for theCI($^3P_1-\,^3P_0$) line at 492 GHz towards two carbon stars, S Sct and R Scl,with detached CO shells of different ages, about 8000 and 2300 years,respectively. The CI($^3P_1-\,^3P_0$) line was detected towards R Scl. The lineintensity is dominated by emission from the detached shell. The detection is ata level consistent with the neutral carbon coming from the fullphotodissociation of all species except CO, and with only limitedphotoionisation of carbon. The best fit to the observed $^{12}$CO and $^{13}$COline intensities, assuming a homogeneous shell, is obtained for a shell mass ofabout 0.002 $M_\odot$, a temperature of about 100 K, and a CO abundance withrespect to H$_2$ of 10$^{-3}$. The estimated CI/CO abundance ratio is about 0.3for the best-fit model. However, a number of arguments point in the directionof a clumpy medium, and a viable interpretation of the data within such acontext is provided.

Detection of CI line emission from the detached CO shell of the AGB star R Sculptoris