Spectral evolution and radial dust transport in the disk of the prototype young eruptive star EX Lup
Description
During the outbursts of EXor stars, the enhanced illumination from the central region heats the inner disk surface within 1 au to a high temperature, permitting pristine amorphous silicate grains to anneal into crystalline ones. The first direct evidence of this process was the discovery of crystalline features in our Spitzer spectrum of the prototype star EX Lup during its 2008 outburst. Our VLTI/MIDI observation during the outburst constrained that the newly formed crystals are in a compact region within 1 au. We analyzed post-outburst Spitzer, VLTI/MIDI, and new VLT/VISIR data and found that the crystalline silicate features faded 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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