This model was developed to describe the chemical evolution of envelopes surrounding carbon-rich asymptotic giant branch (AGB) stars. Named BRAHMA, it represents the conceptual opposite of our earlier model, SHIVA, which focused solely on dust destruction processes. In contrast, BRAHMA emphasizes dust formation, with particular attention given to the synthesis and growth of polycyclic aromatic hydrocarbons (PAHs).

Our primary objectives are:

1. To compile a comprehensive network of chemical reactions governing PAH formation and growth, starting from the simplest building block - the first benzene ring.

2. To integrate this network into a kinetic model and simulate the evolution of PAHs in AGB stellar envelopes.

The model builds upon the work of Isabelle Cherchneff and collaborators, who developed a dynamical framework for AGB envelopes incorporating periodic shock waves. We adopt their physical conditions for our astrochemical modeling. While the initial reaction network was inherited from Cherchneff’s studies as well , we have significantly expanded it by incorporating reactions involving aromatic and non-aromatic hydrocarbons.

We conducted an exhaustive review of the literature, extracting kinetic data for hydrocarbon-related reactions many of which had never before been applied in astrochemistry. This effort remains ongoing, as the vast complexity of carbonaceous molecules makes it impossible to fully map every possible pathway.

Additionally, BRAHMA includes equations for dust growth via accretion and coagulation. The model assumes that the initial seeds for these processes are silicon carbide (SiC) particles, which form earliest in the envelope, preceding the development of aromatic molecules.