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During the outbursts of EXor stars, the enhanced illumination from the central region heat the surface of inner disk within ~1 au to high temperature, permitting pristine amorphous silicate grain to anneal into crystalline ones. The first direct evidence of such crystal formation process was the crystal spectral features detected in our Spitzer spectrum of the prototype star EX Lup itself, during its 2008 outburst. Our follow-up VLTI/MIDI observation during the outburst constrained that the newly formed crystals locate in a compact region within ~1 au. Post-outburst observations with Spitzer, VLTI/MIDI, and VLT/VISIR found the crystal silicate feature to fade with time and finally disappeared within several years after the outburst. We interpret the temporal change of the spectrum with a scenario that the crystalline silicate grains created in the inner region are transported outwards, forming a crystal-rich expanding shell. Our radiative transfer modeling successfully reproduced the features in the post-outburst spectra. The mass of crystal grains in our model is consistent with an origin in the heated inner disk surface. The expanding velocity of the shell is also constrained.
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