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We present Spitzer IRS mid-infrared (5-35 micron) spectra of a completeflux-limited sample (> 3 mJy at 8 micron) of young stellar object (YSO)candidates selected on the basis of their infrared colors in the SerpensMolecular Cloud. Spectra of 147 sources are presented and classified.Background stars (with slope consistent with a reddened stellar spectrum andsilicate features in absorption), galaxies (with redshifted PAH features) and aplanetary nebula (with high ionization lines) amount to 22% of contamination inthis sample, leaving 115 true YSOs. Sources with rising spectra and iceabsorption features, classified as embedded Stage I protostars, amount to 18%of the sample. The remaining 82% (94) of the disk sources are analyzed in termsof spectral energy distribution shapes, PAHs and silicate features. Thepresence, strength and shape of these silicate features are used to infer diskproperties for these systems. About 8% of the disks have 30/13 micron fluxratios consistent with cold disks with inner holes or gaps, and 3% of the disksshow PAH emission. Comparison with models indicates that dust grains in thesurface of these disks have sizes of at least a few \mu\m. The 20 micronsilicate feature is sometimes seen in absence of the 10 micron feature, whichmay be indicative of very small holes in these disks. No significant differenceis found in the distribution of silicate feature shapes and strengths betweensources in clusters and in the field. Moreover, the results in Serpens arecompared with other well-studied samples: the c2d IRS sample distributed over 5clouds and a large sample of disks in the Taurus star-forming region. Theremarkably similar distributions of silicate feature characteristics in sampleswith different environment and median ages - if significant - imply that thedust population in the disk surface results from an equilibrium between dustgrowth and destructive collision processes that are maintained over a fewmillion years for any YSO population irrespective of environment.

A Spitzer Survey of Protoplanetary Disk Dust in the Young Serpens Cloud: How do Dust Characteristics Evolve with Time?