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 for investigating the properties of interstellar dust, especially larger-sized grains. Based on the stellar parameters derived from the SDSS-III/Apache Point Observatory Galaxy Evolution Experiment (APOGEE) spectroscopic survey, we select a large sample of G-type 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 extinctions (relative to $A_{K_S}$, the K _S-band extinction at wavelength λ = 2.16 μm) for the four Wide-field Infrared Survey Explorer (WISE) bands at 3.4, 4.6, 12, and 22 μm, the four Spitzer/Infrared Array Camera bands at 3.6, 4.5, 5.8, and 8 μm, the Spitzer/MIPS24 band at 23.7 μm, and, for the first time, the AKARI/S9W band at 8.23 μm. Our results agree with previous works in that the extinction curve is flat in the ∼3–8 μm 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 $A_{K_S}$, 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.