  We present the first comprehensive, comparative Spitzer/IRS study of the dust
and gas properties of disks around young sun-like stars (K1-M5) and cool
stars/brown dwarfs (M5-M9). The comparison of these two large samples of over
60 sources reveal major differences in the evolution of both the dust and gas
components.We report the first detections of organic molecules in disks around
brown dwarfs. The detection rate statistics and the line flux ratios of HCN and
C2H2 show a striking diference between the two samples, demonstrating a
significant under-abundance of HCN relative to C2H2 in the disk surface of cool
stars. We propose this to originate from the large difference in the
UV-irradiation around the two types of sources. The statistical comparison of
the 10 micron silicate emission features also reveals a significant difference
between the two samples. Cool stars and brown dwarfs show weaker features
arising from more processed silicate grains in the disk atmosphere. These
findings complement previous indications of flatter disk structures and longer
disk lifetimes around cool stars.
