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

We present multiepoch infrared photometry and spectroscopy obtained with warm Spitzer, Subaru, and the Stratospheric Observatory for Infrared Astronomy to assess variability for the young (similar to 20 Myr) 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, while significant nonperiodic variations (peak to peak of similar to 10%-15% relative to the primary star) were detected for the latter. Relative to the Spitzer Infrared Spectrograph spectra taken in 2004, multiepoch mid-infrared spectra reveal no change in either the shape of the prominent 10 mu m solid-state features or the overall flux levels (no more than 20%) for both systems, corroborating the fact that the population of submicron-size grains that produce the pronounced solid-state features is stable over a decadal timescale. We suggest that these submicron-size grains were initially generated in an optically thick clump of debris of millimeter-size vapor condensates resulting from a recent violent impact between large asteroidal or planetary bodies. Because of the shielding from the stellar photons provided by this clump, intense collisions led to an overproduction of fine grains that would otherwise be ejected from the system by radiation pressure. As the clump is sheared by its orbital motion and becomes optically thin, a population of very fine grains could remain in stable orbits until Poynting-Robertson drag slowly spirals them into the star. We further suggest that the 3-5 mu m disk variation around HD 113766A is consistent with a clump/arc of such fine grains on a modestly eccentric orbit in its terrestrial zone.