Adiabatic explosions of polytropic stars are investigated numerically by a three dimensional Smoothed Particle Hydrodynamics solving the energy equation. The evolutions of point thermal explosions are almost spherically symmetric in a global sense, but they are found to be unstable against the Rayleigh-Taylor instability. In the linear stage, the small fluctuations appear on the surface of contact discontinuity and they grow in a local dynamical time scale. The typical unstable wavelength, which grows in the nonlinear stage, is comparable to the thickness of the spherical shell. As a result, we have the porous density structure on the expanding shell; its density contrast amounts to a factor of 4〜8 while the column density varies within a factor of two. These results suggest the clumpiness of the ejecta of the supernova explosion. The accompanying mixing motion in the expanding shell can explain the rapidly rising light curve of SN1987A. Because it mixes up the energy source ^<56>Ni towards the outer layer of supernova.
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