The University of Arizona: DUST EVOLUTION IN THE DENSE INFRARED-DARK CLOUDS

We introduce and present the techniques and results of Mid- and Far-Infrared extinction (MIREX/FIREX) mapping methods of Milky Way infrared-dark clouds (IRDCs) that enable construction of the deepest extinction maps. IRDCs are promising sources to search for massive molecular cores which are assumed to be progenitors of high-mass protoclusters. Understanding dust grain properties in IRDCs is important to determine the initial condition of massive star cluster formation since the density structure of the molecular clouds can only be estimated in terms of absorption opacities and emissivities of dust grains at very cold temperature. In this study, we measure the spectral energy distribution of Galactic background and foreground interstellar medium of the target IRDCs and utilize the information to construct the multiband MIREX/FIREX maps. The maps allow us to investigate dust opacity variation along MIR – FIR wavelength regime where the opacity trends are sensitive to the chemical composition and size distribution of the dust grains. The comparison of the observed dust opacity trend to theoretical models constrains the best assumption of ice mantle formation and dust coagulation in each pixel position of the extinction maps. After cross-checking the trends with other physical properties, we have found evidence of dust grain growth through ice mantle formation and dust coagulation as we traverse denser and colder regions within the IRDCs. The results also indicate that the densest regions in the earliest stage of high-mass star cluster formation potentially have equivalent dust properties with inner protoplanetary disks. We will explore the implication and potential application of the results.