Date of Award

Spring 2002

Document Type

Restricted Thesis

Terms of Use

© 2002 Joanna M. Brown. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts


Physics & Astronomy Department

First Advisor

Tom Millar

Second Advisor

David H. Cohen


The circumstellar envelopes of very evolved, late-type asymptotic giant branch (AGB) stars exhibit complex molecular structure. The most striking of these objects is the carbon star IRC+10216 (CW Leo) where over fifty molecular species, including carbon-chains with up to eleven carbon atoms have been detected. The existing chemical model by Millar, Herbst and Bettens (2000) consists of 3851 reactions involving 407 gas-phase species, including carbon-based molecules containing up to 23 carbon atoms. The envelope of IRC+10216 is now known to contain shells of enhanced density. Previously, all models assumed a spherically symmetric outflow and a 1/r ² density distribution. The dust shells are well correlated with molecular shells and clumps seen in interferometric observations. The model discussed in this paper includes these shells and shows nontrivial changes in the distribution and abundance of the molecules. Inclusion of shells allowed observations to be matched more closely. Using the observed shell positions (Mauron and Huggins 2000), the models matched almost exactly the positions of chemicals, such as HCO ⁺, C₂H, C₄H and HC₅N, recently determined from millimeter observations (Guélin et al. 2000). Another effect of enhanced density shells was the narrowing of the radii over which the peak abundances occurred. 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. The principles of this model can be applied to both carbon-rich and oxygen-rich AGB stars and planetary nebulae, a later evolutionary stage. Better understanding of molecular processes in space could provide insight into the structure of many astronomical objects with complex chemical environments and even into the formation of life on Earth.