CI 609um and CII 158um spectra of IRC +10216

We also report the detection of the 2P3/2-2P1/2 line (at 1900.5369 GHz) from the C^+^ fine structure singlet at the central position and its non-detection at 32 arcsec from the star. The overall picture of the [CI] emission from IRC +10216 agrees with more limited previous studies. The satisfying agreement between the observed and modelled line profiles, with emission at the systemic velocity appearing beyond one beam (13 arcsec HPBW) from the star, rules out that the C^0^ is located in a thin shell.

Given that the bond energy of CO falls only 0.1eV below the ionisation threshold of C^0^, the absence of observable [CII] emission from sightlines beyond a projected distance of ~10^17^cm (>~20-30arcsec) from the star (adopting a distance of 130 pc) does not contradict a scenario where the majority of C^0^ is located between that of CO and C^+^, as expected for an external far-ultraviolet radiation field. This conjecture is also corroborated by a model in which the C^0^ shell is located farther outside, failing to reproduce the [CI] line profiles at intermediate sky-plane distances from the star.

Comparing a photo-chemical model adopted from literature with the simplifying assumption of a constant C^0^ abundance with respect to the H_2_ density (with the 1/r^2^ fall-off of a mass-conserving expansion flow), we constrain the inner boundary of the [CI] emitting shell, located at ~10^16^cm from the star.