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The chemically controlled synthesis of dust in type II-P supernovae

We study the formation of molecules and dust clusters in the ejecta of solar metallicity, Type II-P supernovae using a chemical kinetic approach. We follow the evolution of molecules and small dust cluster masses from day 100 to day 1500 after explosion. We consider stellar progenitors with initial mass of 12, 15, 19 and 25 Msun that explode as supernovae with stratified ejecta. The molecular precursors to dust grains comprise molecular chains, rings and small clusters of silica, silicates, metal oxides, sulphides and carbides, pure metals, and carbon, where the nucleation of silicate clusters is described by a two-step process of metal and oxygen addition. We study the impact of the 56Ni mass on the type and amount of synthesised dust. We predict that large masses of molecules including CO, SiO, SiS, O2, and SO form in the ejecta. We show that the discrepancy between the small dust masses detected at infrared wavelengths some 500 days post-explosion and the larger amounts of dust recently detected with Herschel in supernova remnants can be explained by the non-equilibrium chemistry linked to the formation of molecules and dust clusters in the ejected material. Dust gradually builds up from small (~10^{-5} Msun) to large masses (~5x 10^{-2} Msun) over a 5 yr period after explosion. Subsequent dust formation and/or growth is hampered by the shortage of chemical agents participating in the dust nucleation and the long time scale for accretion. The results highlight the dependence of the dust chemical composition and mass on the amount of 56Ni synthesised during the explosion. This dependence may partly explain the diversity of epochs at which dust forms in supernovae. More generally, our results indicate that type II-P supernovae are efficient but moderate dust producers with an upper limit on the mass of synthesised dust ranging from ~ 0.03 to 0.09 Msun.