2013ApJ...773...30W

In recent years, the wavelength dependence of interstellar extinction from the ultraviolet (UV) to the near- and mid-infrared (IR) has been studied extensively. Although it is well established that the UV/optical extinction law varies significantly among the different lines of sight, it is not clear how IR extinction varies among various environments. In this work, using the color-excess method and taking red giants as the extinction tracer, we determine interstellar extinction A_λ in the four Spitzer/IRAC bands in [3.6], [4.5], [5.8], [8.0] µm (relative to A_Ks, extinction in the Two Micron All Sky Survey (2MASS) K_s band at 2.16 µm) of the Coalsack nebula, a nearby starless dark cloud, based on the data obtained from the 2MASS and Spitzer/GLIMPSE surveys. We select five individual regions across the nebula that span a wide variety of physical conditions ranging from diffuse and translucent to dense environments, as traced by the visual extinction, the Spitzer/MIPS 24 µm emission, and CO emission. We find that A_λ/A_Ks, mid-IR extinction relative to A_Ks, decreases from diffuse to dense environments, which may be explained in terms of ineffective dust growth in dense regions. The mean extinction (relative to A_Ks) is calculated for the four IRAC bands as well and exhibits a flat mid-IR extinction law consistent with previous determinations for other regions. Extinction in the IRAC 4.5 µm band is anomalously high, much higher than that of the other three IRAC bands, and cannot be explained in terms of CO and CO₂ice. Mid-IR extinction in the four IRAC bands has also been derived for four representative regions in the Coalsack Globule 2, which respectively exhibit strong ice absorption, moderate or weak ice absorption, and very weak or no ice absorption. The derived mid-IR extinction curves are all flat, with A_λA/A_Ks increasing with the decrease of the 3.1 µm H₂ O ice absorption optical depth τ_ice.