A Precise Determination of the Mid-Infrared Interstellar Extinction Law Based on the APOGEE Spectroscopic Survey

A precise measure of the mid-infrared interstellar extinction law is crucial to the investigation of the properties of interstellar dust, especially of the grains in the large size end. Based on the stellar parameters derived from the SDSS-III/APOGEE spectroscopic survey, we select a large sample of G- and K-type giants as the tracers of the Galactic mid-infrared extinction. We calculate the intrinsic stellar color excesses from the stellar effective temperatures and use them to determine the mid-infrared extinction for a given line of sight. For the entire sky of the Milky Way surveyed by APOGEE, we derive the extinction (relative to the K$_{\rm S}$ band at wavelength $λ=2.16μ$m) for the four WISE bands at 3.4, 4.6, 12 and 22$μ$m, the four Spitzer/IRAC bands at 3.6, 4.5, 5.8 and 8$μ$m, the Spitzer/MIPS24 band at 23.7$μ$m and for the first time, the AKARI/S9W band at 8.23$μ$m. Our results agree with previous works in that the extinction curve is flat in the ~3--8$μ$m wavelength range and is generally consistent with the $R_V=5.5$ model curve except our determination exceeds the model prediction in the WISE/W4 band. Although some previous works found that the mid-IR extinction law appears to vary with the extinction depth $A_{\rm{K_S}}$, no noticeable variation has been found in this work. The uncertainties are analyzed in terms of the bootstrap resampling method and Monte-Carlo simulation and are found to be rather small.

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