Formation and evolution of dust and molecules in the supernova remnant Cassiopeia A

This PhD thesis addresses the formation of molecules and dust in the ejecta of supernovae (SNe) of Type IIb and in their following reprocessing by shocks in the supernova remnant (SNR), with a focus on the Cassiopeia A (Cas A) remnant. Cas A is a young (∼300 years), close by (3.4 kpc) SNR, where the reverse shock is currently reprocessing the material formed after the SN explosion. Recent observation of dust and molecules with Herschel and ALMA in Cas A, as well as in the young SNR 1987A pose the question whether SNe and SNRs are efficient dust providers to the galaxy. It is not clear how much dust is formed in SNe ejecta, as well as how much is destroyed by the inward reverse shock during the remnant phase. High dust mass are inferred from the reddening of quasars at high redshift, pointing to SNe as the main source of dust at early times. At the moment, theory and observations disagree on the amount of dust formed in SNe ejecta, with lower dust masses observed than predicted by theories.
In this thesis, we want to assess how much dust and molecules form in Cas A ejecta, and how much survive the reprocessing by the reverse shock, in order to infer if SNe Type IIb are dust providers or destroyers.
We first model the SN ejecta chemistry to identify the molecules and dust clusters that form after the explosion and are reprocessed by the reverse shock. We find that Cas A progenitor could have formed large quantities of molecules and dust only in a dense ejecta involving clumps. We then model the impact of the reverse shock on oxygen and carbon-rich ejecta clumps, considering various reverse shock speeds and investigating the post-shock chemistry. We found that the reverse shock destroys the molecules and clusters present in the O-rich clump. CO reforms in the post shock gas with abundances that concur with recent Herschel observations of shocked clumps in Cas A. We derive a dust size distribution for the ejecta of the Cas A progenitor, and investigate the effect of different reverse shock velocities on this dust size distribution. After the clump disruption by the reverse shock, we investigate dust reprocessing by the hot interclump medium. Results show that medium- and large-sized grains in clumps survive the reverse shock and that small dust clusters do not efficiently reform in the shocked gas. This result indicates that the dust formed in the SN ejecta and destroyed by the reverse shock is unable to reform from the gas phase in the SN remnant. Once the grains are released in the hot interclump medium, small grains are quickly destroyed, while only the larger ones survive. Oxide grains are almost completely destroyed, pointing to the inability of SN Type IIb to contribute significantly to the galactic dust budget. Large grains, with radius ∼ 1 micron (such as formed in Type IIP SNe) are required to survive the remnant phase. Carbon and silicon carbide grains are more resistant, and survive even at smaller radii. SNRs with dense clumps and dust grain size distributions including large grains can be significant contributors to the dust budget in the early as well as in the local universe.