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E. Oby, S. D. Doty (Denison University)
Carbon monoxide is an important constiuent in the gas outflows of the circumstellar envelopes (CSEs) surrounding evolved stars. An understanding of these sources requires a comparison of detailed models with observations. Previous modeling work on the gas thermal balance by Sahai and others suggest that cooling by expansion can cause the kinetic temperature to fall below the 2.7K cosmic microwave background radiation (CMBR). Using an approximate treatment of the radiative transfer, these models also predict CO excitation temperatures to drop below the level of the CMBR. We apply a detailed radiative transfer model to this problem in order to further explore the possibility of the physical existence of this phenomenon. We present results and a physical interpretation of the models, as well as a discussion of the potential observability of this effect.