Far-infrared extinction mapping of infrared dark clouds

摘要

Progress in understanding star formation requires detailed observationalconstraints on the initial conditions, i.e. dense clumps and cores in giantmolecular clouds that are on the verge of gravitational instability. Suchstructures have been studied by their extinction of Near-Infrared (NIR) and,more recently, Mid-Infrared (MIR) background light. It has been somewhat moreof a surprise to find that there are regions that appear as dark shadows atFar-Infrared (FIR) wavelengths as long as $\sim$100$\mu m$. Here we developanalysis methods of FIR images from Spitzer-MIPS and Herschel-PACS that allowquantitative measurements of cloud mass surface density, $\Sigma$. The methodbuilds upon that developed for MIR extinction mapping (MIREX) (Butler and Tan2012), in particular involving a search for independent saturated, i.e. veryopaque, regions that allow measurement of the foreground intensity. We focus onthree massive starless core/clumps in IRDC G028.37+00.07, deriving mass surfacedensity maps from 3.5 to 70$\mu m$. A by-product of this analysis ismeasurement of the spectral energy distribution of the diffuse foregroundemission. The lower opacity at 70$\mu m$ allows us to probe to higher $\Sigma$values, up to $\sim1\:{\rm{g\:cm}^{-2}}$ in the densest parts of thecore/clumps. Comparison of the $\Sigma$ maps at different wavelengthsconstrains the shape of the MIR-FIR dust opacity law in IRDCs. We find it ismost consistent with the thick ice mantle models of Ossenkopf and Henning(1994). There is tentative evidence for grain ice mantle growth as one goesfrom lower to higher $\Sigma$ regions.