Modelling the dust and gas outflows from IRC+10 216 - I. Ground-based and airborne observations

Abstract

We have developed a model for the dust and gas envelope of the C star IRC+10 216. Spherical symmetry is assumed, and the model consistently solves the full radiative transfer problem for the rotationally excited far-infrared and submillimetre wavelength CO lines and for the dust continuum. New observations of the CO J=9--8 and 12--11 lines, made with the Kuiper Airborne Observatory, are presented. The model accounts for the first 32 rotational states in the lowest two vibrational levels of CO, and is shown to yield satisfactory fits to both line profiles and spatial maps of the CO J=1--0, 2--1, 3--2, 4--3, 6--5, 7--6, 9--8 and 12--11 lines. The dust model yields a good fit to the spectral energy distribution from the near-IR to millimetre wavelengths, assuming a distance to the star of 170 pc. From the CO model we are able to confirm previous findings that the gas in the outer envelope is heated by the photoelectric effect, and we also find that the mass-loss rate must be of order 5 × 10-5 Mo˙ yr-1, with a gas-to-dust ratio of approximately 220, in order to fit all the CO observations and the spectral energy distribution simultaneously, and to predict accurately the observed wind terminal velocity via radiative acceleration of the dust grains which are momentum-coupled to the gas. The gas temperature distribution is found to be lower than predicted by a simple three-level molecule approach which has been found to work for the envelopes of O-rich asymptotic giant branch stars, but is in good agreement with some previously published models for this source. In contrast with some previously published models, we find no evidence for a recent change in mass-loss rate.