On the physical structure of IRC+10216 Ground-based and Herschel observations of CO and C2H

Context. The carbon-rich asymptotic giant branch star IRC +10 216 undergoes strong mass loss, and quasi-periodic enhancements of the density of the circumstellar matter have previously been reported. The star’s circumstellar environment is a well-studied and complex astrochemical laboratory, in which many molecular species have been proved to be present. CO is ubiquitous in the circumstellar envelope, while emission from the ethynyl (C2H) radical is detected in a spatially confined shell around IRC +10 216. We recently detected unexpectedly strong emission from the N = 4−3, 6−5, 7−6, 8−7, and 9−8 transitions of C2H with the IRAM 30 m telescope and with Herschel/HIFI, which challenges the available chemical and physical models. Aims. We aim to constrain the physical properties of the circumstellar envelope of IRC +10 216, including the effect of episodic mass loss on the observed emission lines. In particular, we aim to determine the excitation region and conditions of C2H to explain the recent detections and to reconcile them with interferometric maps of the N = 1−0 transition of C2H. Methods. Using radiative-transfer modelling, we provide a physical description of the circumstellar envelope of IRC +10 216, constrained by the spectral-energy distribution and a sample of 20 high-resolution and 29 low-resolution CO lines – to date, the largest modelled range of CO lines towards an evolved star. We furthermore present the most detailed radiative-transfer analysis of C2H that has been done so far. Results. Assuming a distance of 150 pc to IRC +10 216, the spectral-energy distribution was modelled with a stellar luminosity of 11300 L⊙ and a dust-mass-loss rate of 4.0 × 10-8 M⊙ yr-1. Based on the analysis of the 20 high-frequency-resolution CO observations, an average gas-mass-loss rate for the last 1000 years of 1.5 × 10-5 M⊙ yr-1 was derived. This results in a gas-to-dust-mass ratio of 375, typical for this type of star. The kinetic temperature throughout the circumstellar envelope is characterised by three power laws: Tkin(r) ∝ r-0.58 for radii r ≤ 9 stellar radii, Tkin(r) ∝ r-0.40 for radii 9 ≤ r ≤ 65 stellar radii, and Tkin(r) ∝ r-1.20 for radii r ≥ 65 stellar radii. This model successfully describes all 49 observed CO lines. We also show the effect of density enhancements in the wind of IRC +10 216 on the C2H-abundance profile, and the close agreement we find of the model predictions with interferometric maps of the C2H N = 1−0 transition and with the rotational lines observed with the IRAM 30 m telescope and Herschel/HIFI. We report on the importance of radiative pumping to the vibrationally excited levels of C2H and the significant effect this pumping mechanism has on the excitation of all levels of the C2H-molecule