Condensation of dust in the ejecta of type II-P supernovae

Aims: We study the production of dust in Type II-P supernova by coupling the gas-phase chemistry to the dust nucleation and condensation phases. We consider two supernova progenitor masses with homogeneous and clumpy ejecta to assess the chemical type and quantity of dust that forms. Grain size distributions are derived as a function of post-explosion time. Methods: The chemistry of the gas phase and the simultaneous formation of dust clusters are described by a chemical network. The formation of key species (CO, SiO) and dust clusters of silicates, alumina, silica, metal carbides and sulphides, pure metals, and amorphous carbon is considered. The master equations describing the chemistry of the nucleation phase are coupled to a dust condensation formalism based on Brownian coagulation. Results: Type II-P supernovae produce dust grains of various chemical compositions and size distributions as a function of time. The grain size distributions gain in complexity with time, are slewed towards large grains, and differ from the usual MRN power-law distribution used for interstellar dust. Gas density enhancements in the form of clumps strongly affect the dust chemical composition and the grain size distributions. Silicates and pure metallic grains are highly dependent on clumpiness. Specifically, clumpy ejecta produce grains over 0.1 micron, and the final dust mass reaches 0.14 Msun. Conversely, carbon and alumina dust masses are controlled by the mass yields of alumina and carbon in the zones where the dust is produced. Several dust components form in the ejecta and the total dust mass gradually builds up over a time span of 3 to 5 years post-outburst. This gradual growth provides a possible explanation for the discrepancy between the small dust masses formed at early post-explosion times and the high dust masses derived from recent observations of supernova remnants.

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.

Molecules in supernova ejecta

The first molecules detected at infrared wavelengths in the ejecta of a Type II supernova, namely SN1987A, consisted of CO and SiO. Since then, confirmation of the formation of these two species in several other supernovae a few hundred days after explosion has been obtained. However, supernova environments appear to hamper the synthesis of large, complex species due to the lack of microscopically-mixed hydrogen deep in supernova cores. Because these environments also form carbon and silicate dust, it is of importance to understand the role played by molecules in the depletion of elements and how chemical species get incorporated into dust grains. In the present paper, we review our current knowledge of the molecular component of supernova ejecta, and present new trends and results on the synthesis of molecules in these harsh, explosive events.