Observational Constraints on The Evolution of Dust in ...

112 Spitzer Survey of Protoplanetary Disk Dust in Serpens
Figure A.1 – IRS spectra of the background stars toward Serpens with the characteristic 10 µm
silicate absorption feature and falling SED. In gray are the original spectra, while in black a binned
version is overplotted. The object number is indicated at the top right of each panel.
Besides these 18 objects, another 8 objects in this sample with falling mid-IR
spectra show the 10 µm silicate feature in absorption (objects 19, 25, 37, 78, 95, 126
were not observed by Oliveira et al. 2009), leading to their classification as background
stars. Alternatively, those eight objects could be highly extincted YSOs, with silicate
features in absorption arising from such extinction. However, using the dense cloud
dust properties, as in Section 4.3.3.5, an extinction of 10 mag would produce a τ 9.7
of only 0.14 (see also Chiar et al. 2007). Since these objects do have silicate features
in absorption, it is understood that they have high extinction (considerably higher
than what is found for the cloud at their location). Thus, it is unlikely that they are
actually highly extincted cloud members. Our working assumption is that these are
background objects. These 26 spectra are shown in Figure A.1.