2013A&A...556A..48S

We investigate the reddening behaviors of young stellar objects (YSOs) in the photometric system of the InfraRed Array Camera (IRAC) onboard the Spitzer Space Telescope. We constructed a couple of model extinction laws for the IRAC wavelength range that are composed of a power-law function and a 9.7µm absorption bump by silicates. We then calculated reddened colors of the YSOs in 3 different evolutionary stages with various masses and disk inclinations by convolving the adopted model spectral energy distributions (SEDs) with the spectral response functions of the IRAC bands for K_S-band extinction values of up to 8 mag. We find that the reddening vectors of YSOs in the [3.6]-[4.5] vs. [5.8]-[8.0] diagrams are considerably SED-dependent and can be highly nonlinear in some cases. Many YSOs show a ``bluening'' (as opposed to the reddening) in the [5.8]-[8.0] color owing to the 9.7µm bump in the extinction law. Our findings imply that dereddening in a color-color or color-magnitude diagram involving the [8.0] band might be highly unreliable unless the SED of the source and the extinction law are at least approximately known.