  The C-rich AGB star IRC+10216 undergoes strong mass loss, and quasi-periodic
density enhancements in the circumstellar matter have been reported. CO is
ubiquitous in the CSE, while CCH emission comes from a spatially confined
shell. With the IRAM 30m telescope and Herschel/HIFI, we recently detected
unexpectedly strong emission from the CCH N=4-3, 6-5, 7-6, 8-7, and 9-8
transitions, challenging the available chemical and physical models. We aim to
constrain the physical properties of IRC+10216's CSE, including the effect of
episodic mass loss on the observed emission. In particular, we aim to determine
the excitation region and conditions of CCH and to reconcile these with
interferometric maps of the N=1-0 transition. Via radiative-transfer modelling,
we provide a physical description of the CSE, constrained by the SED and a
sample of 20 high-resolution and 29 low-resolution CO lines. We further present
detailed radiative-transfer analysis of CCH. Assuming a distance of 150pc, the
SED is modelled with a stellar luminosity of 11300Lsun and a dust-mass-loss
rate of 4.0\times10^{-8}Msun/yr. Based on the analysis of 20 high resolution CO
observations, an average gas-mass-loss rate for the last 1000yrs of
1.5\times10^{-5}Msun/yr is derived. This gives a gas-to-dust-mass ratio of 375,
typical for an AGB star. The gas kinetic temperature throughout the CSE is
described by 3 powerlaws: it goes as r^{-0.58} for r<9R*, as r^{-0.40} for
9<=r<=65R*, and as r^{-1.20} for r>65R*. This model successfully describes all
49 CO lines. We show the effect of wind-density enhancements on the
CCH-abundance profile, and the good agreement of the model with the CCH N=1-0
transition and with the lines observed with the 30m telescope and HIFI. We
report on the importance of radiative pumping to the vibrationally excited
levels of CCH and the significant effect this has on the excitation of all
levels of the CCH-molecule.
