Detection of CI line emission from the detached CO shell of the AGB star
  R Sculptoris
  Stars on the asymptotic giant branch (AGB) lose substantial amounts of
matter, to the extent that they are important for the chemical evolution of,
and dust production in, the universe. The mass loss is believed to increase
gradually with age on the AGB, but it may also occur in the form of bursts,
possibly related to the thermal pulsing phenomenon. Detached, geometrically
thin, CO shells around carbon stars are good signposts of brief and intense
mass ejection. We aim to put further constraints on the physical properties of
detached CO shells around AGB stars. The photodissociation of CO and other
carbon-bearing species in the shells leads to the possibility of detecting
lines from neutral carbon. We have therefore searched for the
CI($^3P_1-\,^3P_0$) 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 line
intensity is dominated by emission from the detached shell. The detection is at
a level consistent with the neutral carbon coming from the full
photodissociation of all species except CO, and with only limited
photoionisation of carbon. The best fit to the observed $^{12}$CO and $^{13}$CO
line intensities, assuming a homogeneous shell, is obtained for a shell mass of
about 0.002 $M_\odot$, a temperature of about 100 K, and a CO abundance with
respect to H$_2$ of 10$^{-3}$. The estimated CI/CO abundance ratio is about 0.3
for the best-fit model. However, a number of arguments point in the direction
of a clumpy medium, and a viable interpretation of the data within such a
context is provided.
