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We present multi-epoch infrared photometry and spectroscopy obtained withwarm Spitzer, Subaru and SOFIA to assess variability for the young ($\sim$20Myr) and dusty debris systems around HD 172555 and HD 113766A. No variations(within 0.5%) were found for the former at either 3.6 or 4.5 $\mu$m, whilesignificant non-periodic variations (peak-to-peak of $\sim$10-15% relative tothe primary star) were detected for the latter. Relative to the Spitzer IRSspectra taken in 2004, multi-epoch mid-infrared spectra reveal no change ineither the shape of the prominent 10 $\mu$m solid-state features or the overallflux levels (no more than 20%) for both systems, corroborating that thepopulation of sub-$\mu$m-sized grains that produce the pronounced solid-statefeatures is stable over a decadal timescale. We suggest that thesesub-$\mu$m-sized grains were initially generated in an optically thick clump ofdebris of mm-sized vapor condensates resulting from a recent violent impactbetween large asteroidal or planetary bodies. Because of the shielding from thestellar photons provided by this clump, intense collisions led to anover-production of fine grains that would otherwise be ejected from the systemby radiation pressure. As the clump is sheared by its orbital motion andbecomes optically thin, a population of very fine grains could remain in stableorbits until Poynting-Robertson drag slowly spirals them into the star. Wefurther suggest that the 3-5 $\mu$m disk variation around HD 113766A isconsistent with a clump/arc of such fine grains on a modestly eccentric orbitin its terrestrial zone.

Mid-infrared Studies of HD 113766 and HD 172555: Assessing Variability in the Terrestrial Zone of Young Exoplanetary Systems