Milky Way Mid-Infrared Spitzer Spectroscopic Extinction Curves: Continuum and Silicate Features

We measured the mid-infrared (MIR) extinction using Spitzer photometry and spectroscopy (3.6–37 μ m) for a sample of Milky Way sight lines (mostly) having measured ultraviolet extinction curves. We used the pair method to determine the MIR extinction that we then fit with a power law for the continuum and modified Drude profiles for the silicate features. We derived 16 extinction curves having a range of A ( V ) (1.8–5.5) and R ( V ) values (2.4–4.3). Our sample includes two dense sight lines that have 3 μ m ice feature detections and weak 2175 Å bumps. The average A ( λ )/ A ( V ) diffuse sight-line extinction curve we calculate is lower than most previous literature measurements. This agrees better with literature diffuse dust grain models, though it is somewhat higher. The 10 μ m silicate feature does not correlate with the 2175 Å bump, for the first time providing direct observational confirmation that these two features arise from different grain populations. The strength of the 10 μ m silicate feature varies by ∼2.5 and is not correlated with A ( V ) or R ( V ). It is well fit by a modified Drude profile with strong correlations seen between the central wavelength, width, and asymmetry. We do not detect other features with limits in A ( λ )/ A ( V ) units of 0.0026 (5–10 μ m), 0.004 (10–20 μ m), and 0.008 (20–40 μ m). We find that the standard prescription of estimating R ( V ) from C × E ( K s − V )/ E ( B − V ) has C = −1.14 and a scatter of ∼7%. Using the IRAC 5.6 μ m band instead of K s gives C = −1.03 and the least scatter of ∼3%.