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

TL;DR

This study precisely measures the mid-infrared interstellar extinction law across the entire sky using APOGEE data, revealing a generally flat extinction curve consistent with models but with some deviations, and finds no significant variation with extinction depth.

Contribution

It provides the first comprehensive mid-infrared extinction law for the entire sky based on APOGEE data, with detailed analysis of uncertainties and comparison to existing models.

Findings

Extinction curve is flat between 3-8 μm.

Results are consistent with R_V=5.5 model, with some deviations.

No significant variation with extinction depth found.

Abstract

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 $\lambda=2.16\mu$m) for the four WISE bands at 3.4, 4.6, 12 and 22$\mu$m, the four Spitzer/IRAC bands at 3.6, 4.5, 5.8 and 8$\mu$m, the Spitzer/MIPS24 band at 23.7$\mu$m and for the first time, the AKARI/S9W band at 8.23$\mu$m. Our results agree with previous works in that the extinction curve is flat in the ~3--8$\mu$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.