  We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC 7160.
The sample includes the high- and intermediate-mass stars with MIPS 24 microns
excess, the only known active accretor in the 12 Myr-old cluster NGC 7160, and
19 low-mass stars with disks in the 4 Myr-old cluster Tr 37. We examine the 10
microns silicate feature, present in the whole sample of low-mass star and in 3
of the high- and intermediate-mass targets, and we find that PAH emission is
detectable only in the Herbig Be star. We analyze the composition and size of
the warm photospheric silicate grains by fitting the 10 microns silicate
feature, and study the possible correlations between the silicate
characteristics and the stellar and disk properties (age, SED slope, accretion
rate, spectral type). We find indications of dust settling with age and of the
effect of turbulent enrichment of the disk atmosphere with large grains.
Crystalline grains are only small contributors to the total silicate mass in
all disks, and do not seem to correlate with any other property, except maybe
binarity. We also observe that spectra with very weak silicate emission are at
least 3 times more frequent among M stars than among earlier spectral types,
which may be an evidence of inner disk evolution. Finally, we find that 5 of
the high- and intermediate-mass stars have SEDs and IRS spectra consistent with
debris disk models involving planet formation, which could indicate debris disk
formation at ages as early as 4 Myr.
