1998A&A...338..491G

¹²CO and ¹³CO J=6-5 observations of IRC +10 216 with the JCMT are presented. A spherically symmetric radiative transfer code is used to model these and other CO observations of the carbon star IRC +10 216/CW Leo. Compared to previously published model calculations a much larger set of observational data is used as constraints; on-source ¹²CO and ¹³CO J=1-0 up to J=6-5 and mapping data taken with various telescopes, most of which are obtained from the literature. The gas temperature in the envelope is calculated taking into account heating and cooling. The heating by dust-gas collisions and various other parameters (such as luminosity over distance squared) are constrained from our previous modeling of the circumstellar dust shell. Photoelectric heating is taken into account. A grid of models is calculated with the following parameters: luminosity (in the range 10000-30000, in steps of 5000L_☉), mass loss rate, dust-to-gas ratio, dust opacity and CO abundance. For each of the considered luminosities a good fit to the on-source data can be obtained. A comparison with CO J=1-0 data obtained from the literature points towards a preferred luminosity of 10-15000L_☉. Notwithstanding the overall good agreement, there remain discrepancies. The different observed ¹²CO J=3-2 observations appear to be always larger than the model predictions. The observed ¹³CO J=6-5 is almost flat-topped, while the model gives a slight double-peaked profile. There is a large discrepancy with the single existing ¹²CO J=7-6 observation. The best fitting models (for each of the considered luminosities) cannot accommodate the more extended emission seen in the mapping data. This is not due to an underestimate of the photoelectric effect. To fit the data for radial offsets >50" the mass loss rate must be a factor of 5 higher in the outer envelope. Because the various sets of data for offsets >150" are not consistent with each other it is unclear if the enhancement in the mass loss rate extends beyond that radius. Visibility curves are calculated for comparison with future interferometric observations. These appear to be insensitive to luminosity and mass loss variations but should be good tracers of the geometry of the CO shell. A comparison is made between the mass loss rates and dust-to-gas ratios derived from the CO modelling and those derived from our previous dust modelling. To do this we make use of the relation {dot}(M) v_∞=τ_F(L)/(c)(1-(1)/(Γ)), which is valid for radiation pressure driven outflows. The best agreement is obtained for the model with 15 000L_☉. This agrees well with the luminosity range 7700-12500L_☉ based on the period-luminosity relation for carbon miras. In summary, we conclude that the likely luminosity of IRC +10 216 is between 10000 and 15000L_☉ and that its distance is between 110 and 135pc. The present-day mass loss rate is (1.5±0.3)x10^–5M_☉/yr and the gas-to-dust ratio is a 700±100. The dust opacity at 60µm is found to be of order of 250cm²/g. The CO abundance is 1.1x10^–3 relative to H₂.