  Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we
observed multiple epochs of 11 actively accreting T Tauri stars in the nearby
Taurus-Auriga star forming region. In total, 88 low-resolution mid-infrared
spectra were collected over 1.5 years in Cycles 2 and 3. The results of this
multi-epoch survey show that the 10 um silicate complex in the spectra of two
sources - DG Tau and XZ Tau - undergoes significant variations with the
silicate feature growing both weaker and stronger over month- and year-long
timescales. Shorter timescale variations on day- to week-long timescales were
not detected within the measured flux errors. The time resolution coverage of
this data set is inadequate for determining if the variations are periodic.
Pure emission compositional models of the silicate complex in each epoch of the
DG Tau and XZ Tau spectra provide poor fits to the observed silicate features.
These results agree with those of previous groups that attempted to fit only
single-epoch observations of these sources. Simple two-temperature, two-slab
models with similar compositions successfully reproduce the observed variations
in the silicate features. These models hint at a self-absorption origin of the
diminution of the silicate complex instead of a compositional change in the
population of emitting dust grains. We discuss several scenarios for producing
such variability including disk shadowing, vertical mixing, variations in disk
heating, and disk wind events associated with accretion outbursts.
