Recalescence during crystallization of stardust: Resolution of the amorphous interstellar medium paradox

Most of the dust that coalesces to form planetary systems initially condenses in the outflows of dying intermediate mass (~1-8M_sol) asymptotic giant branch, or AGB stars. Historically, observations of broad smooth features in the 10-μm region suggested that dust in circumstellar regions, and in the interstellar medium (ISM), was mostly amorphous rather than crystalline. With improved space telescope capabilities, crystalline silicates were discovered in the circumstellar regions around both young and old stars, although they remain largely undetected in the ISM. Despite intensive study the precise conditions that lead to the formation of crystalline silicates are still unknown, and their absence in the ISM remains problematic. Here we show that recalescence (spontaneous reheating) of rapidly crystallizing dust can explain the formation and apparent disappearance of crystalline silicates in space. We have documented recalescence in rapidly crystallizing Mg-rich silicate melts, with local heating at the crystallization front exceeding 160˚C in some cases. In circumstellar dust shells, amorphous grains with similar compositions condense at temperatures near their glass transition, and if they crystallize, they will recalesce. The higher temperature (T) of newly crystallized dust allows crystalline spectral features to be seen, because flux emitted depends on T⁴. After cooling to ambient temperature, crystalline spectral features in the ISM are concealed by volumetrically dominant amorphous dust. Our results explain the existence of crystalline silicate pre-solar grains, which originate around evolved stars, and have implications for radiative transfer modeling and hydrodynamics of dusty environments, which are sensitive to small variations in optical properties.