Formation of dust grains in the ejecta of SN 1987A. II
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Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolo metric light curve around its maximum and the early emergence of X-rays and γ-rays, results in the sequential formation of Al2O3, MgSiO3 and Fe3O4 grains respectively in the ejecta at 1.0 M☉ ≤ Mr ≤4.4M☉ as the gas cools down. In the inner region at Mr ≤ 1.0 M☉, on the other hand, the latent heat deposited during the grain growth retards the formation of dust grains. The observed enhancement of 10 μm flux at ~day 465 after the explosion being assigned to the thermal radiation from Al2O3 grains formed in the ejecta, MgSiO3 grains start to form at ~day 550, and Fe3O4 grains at ~day 620, and the grains cease to form at ~day 730 in the region at 1.0 M☉ ≤ Mr ≤ 4.4 M☉. The total mass of dust grains formed in this region is ~0.23 M☉. The radii of grains newly formed in the ejecta are typically ~10 Å for Al2O3 and Fe3O4, and ~70 Å forMgSiO3 grains. The calculated grain radius smaller than 0.01 μm satisfies the constraint on the grain size in the ejecta imposed by the analysis of the blue-shifted optical lines. Comparing the observed infrared light curve with the calculated one based on both the condensation calculation and the observed 10 μm flux, we suggest that: 1) The condensation efficiency of dust grains in the ejecta is greater than 0.3, or the clumps occupies the fraction of at least ~20% of the ejecta on the average. 2) The grain number density in the clumps is less than 5 times that of the uniform ejecta. 3) The observed infrared luminosity is attributed to the thermal radiation from dust grains in the optically thick part of the ejecta.
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