Detailed Chemical Modeling of the Circumstellar Envelopes of Carbon Stars: Application to IRC +10216

Abstract

We have constructed a detailed chemical model for the circumstellar envelope surrounding the nearby, carbon-rich star IRC +10216. This model extends previous approaches for the photochemistry. In particular, we consider the photodissociation of CO using the Sobolev approximation to study the effect of CO self-shielding, include H-2 shielding, treat the effects of source geometry and continuum radiative transfer through dust, and construct a large, detailed gas phase chemical network to probe the effects of these processes on abundance variations of selected species. We then compare our results with previous models and observations to assess their validity and the necessity of including this level of detail. In general, we find that although CO self-shielding is important, one cannot ignore the effect of continuum radiative transfer. This can change the CO dissociation radius by about 30%. The change in the CO dissociation radius can propagate chemical differences into the chemical network. We find that our model predictions can reproduce the column densities inferred from observations as well as or better than previous comprehensive models. Our model also predicts the atomic carbon abundance peak to be significantly closer to the central source than previous photochemical models-a result consistent with strong C I lines observed by Keene et al.