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In recent years the wavelength dependence of interstellar extinction from theultraviolet (UV), optical, through the near- and mid-infrared (IR) has beenstudied extensively. Although it is well established that the UV/opticalextinction law varies significantly among the different lines of sight, it isnot clear how the IR extinction varies among various environments. In thiswork, using the color-excess method and taking red giants as the extinctiontracer, we determine the interstellar extinction Alambda in the fourSpitzer/IRAC bands of the Coalsack nebula, a nearby starless dark cloud, basedon the data obtained from the 2MASS and Spitzer/GLIMPSE surveys. We select fiveindividual regions across the nebula that span a wide variety of physicalconditions, ranging from diffuse, translucent to dense environments, as tracedby the visual extinction, the Spitzer/MIPS 24micron emission, and CO emission.We find that Alambda/AKs, the mid-IR extinction relative to AKs, decreases fromdiffuse to dense environments, which may be explained in terms of ineffectivedust growth in dense regions. The mean extinction (relative to AKs) iscalculated for the four IRAC bands as well, which exhibits a flat mid-IRextinction law, consistent with previous determinations for other regions. Theextinction in the IRAC 4.5micron band is anomalously high, much higher thanthat of the other three IRAC bands. It cannot be explained in terms of CO andCO2 ices. The mid-IR extinction in the four IRAC bands have also been derivedfor four representative regions in the Coalsack Globule 2 which respectivelyexhibit strong ice absorption, moderate or weak ice absorption, and very weakor no ice absorption. The derived mid-IR extinction curves are all flat, withAlambda/AKs increasing with the decrease of the H2O ice absorption opticaldepth.

The Mid-Infrared Extinction Law and its Variation in the Coalsack Nebula