Molecules and dust in Cassiopeia A

Abstract

We study the dust evolution in the supernova remnant Cassiopeia A. We follow the processing of dust grains that formed in the Type II-b supernova ejecta by modelling the sputtering of grains. The dust is located in dense ejecta clumps that are crossed by the reverse shock. We also investigate further sputtering in the inter-clump medium gas once the clumps have been disrupted by the reverse shock. The dust evolution in the dense ejecta clumps of Type II-P supernovae and their remnants is also explored. We study oxygen-rich clumps that describe the oxygen core of the ejecta, and carbon-rich clumps that correspond to the outermost carbon-rich ejecta zone. We consider the various dust components that form in the supernova, several reverse shock velocities and inter-clump gas temperatures, and derive grain-size distributions and masses for the dust as a function of time. Both non-thermal sputtering within clumps and thermal sputtering in the inter-clump medium gas are studied. We find that non-thermal sputtering in the clumps is important for all supernova types and accounts for reducing the grain population by ~ 40% to 80% in mass, depending on the clump gas over-density, the grain type and size, and the shock velocity in the clump. A Type II-b SN forms small grains that are sputtered within the clumps and in the inter-clump medium. For Cas A, silicate grains do not survive thermal sputtering in the inter-clump medium, while alumina, silicon carbide, and carbon dust may survive in the remnant. Our derived masses of currently processed silicate, alumina and carbon grains agree well with the values derived from the observations of warm dust, and seem to indicate that the dust is currently being processed within clumps by non-thermal sputtering. Out of the ~ 0.03M⊙ of dust formed in the ejecta, between 30% and 60% of this mass is present today in Cas A, and only 6% to 11% of the initial mass will survive the remnant phase. Grains formed in Type II-P supernovae are larger and better survive their journey in the remnant with mass fractions of surviving dust in the range 14−45%. For very dense ejecta, which describe the ejected material in supernovae such as SN1987A, non-thermal sputtering is inefficient within the clumps and dust survival efficiencies in the remnant range between 42% to 98% in mass. For the SN1987A model, the derived surviving dust mass is in the range ~ 0.06−0.13M⊙. This type of supernovae with dense ejecta may then efficiently provide galaxies with dust. Specifically, silicate grains over 0.1 μm and other grains over 0,05μm survive thermal sputtering in the remnant. Therefore, pre-solar grains of supernova origin that are found in meteorites, in particular silicate and alumina grains, possibly form in the dense ejecta clumps of Type II-P supernovae.