IRC +10 216 revisited. II. The circumstellar CO shell

(12) CO and (13) CO 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 (12) CO and (13) CO 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 10 000 - 30 000, in steps of 5 000 {L_sun}), 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 10-15 000 {L_sun}. Notwithstanding the overall good agreement, there remain discrepancies. The different observed (12) CO J = 3-2 observations appear to be always larger than the model predictions. The observed (13) CO 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 (12) CO 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)/(Gamma )right ), which is valid for radiation pressure driven outflows. The best agreement is obtained for the model with 15 000 {L_sun}. This agrees well with the luminosity range 7 700-12 500 {L_sun} based on the period-luminosity relation for carbon miras. In summary, we conclude that the likely luminosity of IRC +10 216 is between 10 000 and 15 000 {L_sun} and that its distance is between 110 and 135 pc. The present-day mass loss rate is (1.5 +/- 0.3) x 10(-5) M_sun yr(-1) and the gas-to-dust ratio is a 700 +/- 100. The dust opacity at 60 mu m is found to be of order of 250 cm(2) gr(-1) . The CO abundance is 1.1 x 10(-3) relative to H_2. The James Clerk Maxwell Telescope is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, the Netherlands Organization for Scientific Research, and the National Research Council of Canada.