Molecules and dust in Cas A: I - Synthesis in the supernova phase and processing by the reverse shock in the clumpy remnant, A&A 564

Aims. We study the chemistry of the Type IIb supernova ejecta that led to the Cas A supernova remnant to assess the chemical type and quantity of dust that forms and evolves in the remnant phase. We later model a dense oxygen-rich ejecta knot that is crossed by the reverse shock in Cas A to study the evolution of the clump gas phase and the possibility to reform dust clusters in the post-reverse shock gas. Methods. The chemistry is described by a chemical network that includes all possible processes efficient at high gas temperatures and densities. The formation of key bimolecular species (e.g., CO, SiO) and dust clusters of silicates, alumina, silica, metal carbides and sulphides, pure metals, and amorphous carbon is considered. A set of stiff, coupled, ordinary, differential equations is solved for the conditions pertaining to both the SN ejecta and the post-reverse shock gas. Results. We find that the ejecta of Type IIb SNe are unable to form large amounts of molecules and molecular clusters that are precursors to dust grains as opposed to their Type II-P counterparts because of their diffuse ejecta. The ejecta gas density needs to be increased by several orders of magnitude to allow the formation of dust clusters. We show that the chemical composition of the dust clusters that form changes drastically and gains in chemical complexity with increasing gas density. Hence, the ejecta of the Cas A supernova progenitor must have been in the form of dense clumps to account for the dust chemical composition and masses inferred from infrared observations of Cas A. As for the impact of the reverse shock on dense ejecta clumps, we show that the ejecta molecule