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

Abstract

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.

17 pages, 15 figures; accepted for publication in Astronomy & Astrophysics

References in corpus (9)

• Radiative transfer in very optically thick circumstellar disks
• Photoprocesses in protoplanetary disks
• A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
• Density-enhanced gas and dust shells in a new chemical model for IRC+10216
• Warm water vapour in the sooty outflow from a luminous carbon star
• The circumstellar envelope of IRC+10216 from milli-arcsecond to arcmin scales
• A spectral line survey in the 2 mm and 1.3 mm windows toward the carbon rich envelope of IRC +10216
• Discovery of multiple dust shells beyond 1 arcmin in the circumstellar envelope of IRC+10216 using Herschel/PACS
• Molecular shells in IRC+10216: Evidence for non-isotropic and episodic mass loss enhancement

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