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Infrared photometry and spectroscopy covering a time span of a quartercentury are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both areisolated Herbig Ae stars that exhibit signs of active accretion, includingdriving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 wasfound to be relatively quiescent photometrically in its inner disk region, withthe exception of a major increase in emitted flux in a broad wavelength regioncentered near 3 microns in 2002. In contrast, HD 31648 has exhibited sporadicchanges in the entire 3-13 micron region throughout this span of time. In bothstars the changes in the 1-5 micron flux indicate structural changes in theregion of the disk near the dust sublimation zone, possibly causing itsdistance from the star to vary with time. Repeated thermal cycling through thisregion will result in the preferential survival of large grains, and anincrease in the degree of crystallinity. The variability observed in theseobjects has important consequences for the interpretation of other types ofobservations. For example, source variability will compromise models based oninterferometry measurements unless the interferometry observations areaccompanied by nearly-simultaneous photometric data.

Variability of Disk Emission in Pre-Main Sequence and Related Stars. I. HD 31648 and HD 163296 - Isolated Herbig Ae Stars Driving Herbig-Haro Flows