  We present a high spatial resolution, 10-20 micron survey of 65 T Tauri
binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m
telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks
around the individual stellar components in these binary systems, this study
increases the number of binaries with spatially resolved measurements at 10
micron by a factor of ~5. Combined with resolved near-infrared photometry and
spectroscopic accretion diagnostics, we find that ~10% of stars with a
mid-infrared excess do not appear to be accreting. In contrast to an actively
accreting disk system, these passive disks have significantly lower
near-infrared colors that are, in most cases, consistent with photospheric
emission, suggesting the presence of an inner disk hole. In addition, there
appears to be a spectral type/mass dependence associated with the presence of a
passive disk, with all passive disks occurring around M type stars. The
possibility that the passive disks are caused by the presence of an as yet
undetected companion at a small separation (0.3-3 AU) is possible for any
individual system, however, it cannot account for the spectral type dependence
of the passive disk sample as a whole. We propose that these passive disks
represent a subset of T Tauri stars that are undergoing significant disk
evolution. The fraction of observed passive disks and the observed spectral
type dependence can both be explained by models of disk evolution that include
disk photoevaporation from the central star. (abridged).
