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abstract
We have studied the thermal balance in the circumstellar envelopes of mass-losing, carbon-rich evolved stars. Line cooling is dominated in most parts of these envelopes by rotational emission from the CO molecule. We explore the possible values for mass loss rate, CO abundance and dust parameters by solving the radiative transfer self-consistently with the thermal balance. The radiative transfer is calculated by using a Monte Carlo method. We apply our model to IRC+10216, for which we use published and new (sub)millimeter wavelength observations of various CO transitions to determine our model parameters. In particular, high spatial resolution observations of the moderately high-excitation submillimeter lines are used to put important constraints on the temperature distribution in the inner portion of the envelope around radii of $10^{16}$ cm. In light of the available data, our calculations do not support the existence of a very high temperature ($\simgreat 500 $ K) region extending out to $10^{16}$ cm, as has been previously suggested.
