Observational Constraints on The Evolution of Dust in ...

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A Spitzer Survey of Protoplanetary
Disk Dust in the
Young Serpens Cloud:
How do Dust Characteristics
Evolve with Time?
We present Spitzer InfraRed Spectrograph (IRS) mid-infrared (5–35 µm) spectra of a complete
flux-limited sample (≥ 3 mJy at 8 µm) of young stellar object (YSO) candidates selected
on the basis of their infrared colors in the Serpens Molecular Cloud. Spectra of 147 sources
are presented and classified. Background stars (with slope consistent with a reddened stellar
spectrum and silicate features in absorption), galaxies (with redshifted polycyclic aromatic
hydrocarbon (PAH) features), and a planetary nebula (with high ionization lines) amount
to 22% of contamination in this sample, leaving 115 true YSOs. Sources with rising spectra
and ice absorption features, classified as embedded Stage I protostars, amount to 18% of
the sample. The remaining 82% (94) of the disk sources are analyzed in terms of spectral
energy distribution shapes, PAHs, and silicate features. The presence, strength, and shape
of these silicate features are used to infer disk properties for these systems. About 8% of
the disks have 30/13 µm flux ratios consistent with cold disks with inner holes or gaps, and
3% of the disks show PAH emission. Comparison with models indicates that dust grains
in the surface of these disks have sizes of at least a few µm. The 20 µm silicate feature is
sometimes seen in the absence of the 10 µm feature, which may be indicative of very small
holes in these disks. No significant difference is found in the distribution of silicate feature
shapes and strengths between sources in clusters and in the field. Moreover, the results in
Serpens are compared with other well-studied samples: the c2d IRS sample distributed over
five clouds and a large sample of disks in the Taurus star-forming region. The remarkably