  We present mid-infrared spectra between 5.2 and 38 microns for 26
disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained
with the Infrared Spectrograph onboard the Spitzer Space Telescope. We find
clear evidence for changes in the spectral characteristics of dust emission
between the early (B+A) and late-type (K+M) infrared excess stars. The
early-type members exhibit featureless continuum excesses that become apparent
redward of ~8 microns. In contrast, 10 and 20 micron silicate features are
present in all but one of the late-type excess members of Upper Scorpius. The
strength of silicate emission among late-type Upper Scorpius members is
spectral type dependent, with the most prominent features being associated with
K5-M2 type stars. By fitting the spectral energy distributions (SED) of a
sample of low-mass stars with accretion disk models, we find that the SEDs are
consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU.
Complementary high resolution optical spectra for the Upper Scorpius excess
stars were examined for signatures of gaseous accretion. Of the 35 infrared
excess stars identified in Upper Scorpius, only 7 (all late-type) exhibit
signatures of accretion. Mass accretion rates for these stars range from ~1e-11
to 1e-9 solar masses/yr. Compared to Class II sources in Taurus-Auriga, the
disk population in Upper Scorpius exhibits reduced levels of near and
mid-infrared excess emission and an order of magnitude lower mass accretion
rates. These results suggest that the disk structure has changed significantly
over the 2-4 Myr in age separating these two stellar populations. The ubiquity
of depleted inner disks in the Upper Scorpius excess sample implies that such
disks are a common evolutionary pathway that persists for some time.
