Modelling enhanced density shells in the circumstellar envelope of IRC + 10216

The circumstellar envelopes of very evolved, late-type asymptotic giant branch stars exhibit complex molecular structure. The most striking of these objects is the carbon star IRC +10216 (CW Leo), where over 50 molecular species, including carbon chains with up to nine carbon atoms, have been detected. The envelope of IRC +10216 is now known to contain dust shells of enhanced density which are well correlated with molecular shells seen in interferometric observations. The inclusion of shells, based on observed parameters, into a chemical model allows observations to be matched more closely. Another effect of enhanced density shells is the narrowing of the radii over which the peak abundances occur. Previous models tended to have the molecules spread over a larger band of radii than observations indicated. Thus, the effects of enhanced density shells are extremely important for understanding the circumstellar environments of stars such as IRC +10216.