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Appendix A: On the discrepancy between Spitzer and WISE data

The WISE mission used four IR bands (3.4, 4.6, 12, and 22 μm, W1, W2, W3 and W4 respectively) to survey the whole sky. On the other hand, the Spitzer Space Telescope observed several star-forming regions with its instruments IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70 and 160 μm, the last two not used in this study). IRAC1, IRAC2 and MIPS1 bands are therefore comparable to W1, W2 and W4, respectively. We computed the ratio of measured fluxes (as WISE_band/IRAC_band) as a function of IRAC_band flux, where band corresponds to each of the comparisons mentioned earlier. The results are shown in Fig. A.1: W1 and W2 are compatible with the corresponding IRAC photometry, whereas W4 deviates significantly from MIPS1 measurements. We found this effect to be more important for weaker fluxes, which could indicate two different issues: (1) a problem with WISE flux estimates for unresolved sources, specially important for confused regions; or (2) an important influence of extended background emission. Figures A.1 and A.2 illustrate these two problems in the case of the IC 348 young stellar cluster. IC 348 is a rather dense young cluster with a high and variable background level. WISE fluxes in the W3 and W4 bands were found to be incompatible with the Spitzer ones. The full width at half-maximum (FWHM) of the W4 PSF is 11′′  (WISE All Sky Release Explanatory Supplement), considerably larger than the 6′′  FWHM of the MIPS1 PSF (MIPS Instrument Handbooks), resulting in a higher level of confusion and a greater sensitivity to variable extended background. Figure A.3 shows an example of a problematic source within IC 348. If we trust the WISE photometry blindly, a mid-IR excess is found at 12 and 22 μm. The corresponding Spitzer photometry demonstrates that no such excess exists, and that the source most likely does not harbor a disk.

	Fig. A.1 Differences between WISE and Spitzer data as a function of Spitzer fluxes for the three comparable bands. Only WISE detections with S / N > 5 are plotted in each band.
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	Fig. A.2 Three-color composite images of IC 348 as seen by Spitzer (left, blue: IRAC1, green: IRAC2, and red: MIPS1) and WISE (right, blue: W1, green: W2, and red: W4). The blue dots marks the position of 2MASS J03443274+3208374, which SED is shown in Fig. A.3.
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	Fig. A.3 SED of 2MASS J03443274+3208374, a source located in a region of high density and variable extended emission (see Fig. A.2).
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© ESO, 2013