Physical Parameters of the IRC +10216 Circumstellar Envelope: New Constraints from Submillimeter Observations

Abstract

We have studied the temperature distribution and other physical properties of the circumstellar envelope of the prototypical high mass-loss carbon star IRC + 10216 by calculating the thermal balance and the radiative transfer in the envelope self-consistently. Cooling is dominated by CO line emission and adiabatic expansion, and heating by dust-gas collisions throughout most of the envelope. Heating by the grain photoelectric effect is important in the outer part of the envelope. The radiative transfer is calculated by using a Monte Carlo method. The mass-loss rate, the CO abundance, the dust-gas momentum transfer efficiency, and the distance to the source are free parameters in our model. These physical parameters are constrained by the comparison of our model results with the observations of various (CO)-C-12 and (CO)-C-13 lines. In particular, recent submillimeter-wavelength observations of moderately high excitation transitions, such as the J = 6 --> 5 line, put very important constraints on the temperature distribution in the inner part of the envelope, and they do not support the presence of very high temperatures (T similar to 500-1000 K) in the inner part of the envelope (at about 5 x 10(15) cm from the central star) suggested by a previous study. We also find that a mass-loss rate of 3.25 x 10(-5) M. yr(-1) and a distance of 150 pc provide the best agreement between our model results and observations.