An infrared through radio study of the properties and evolution of IRDC clumps

We examine the physical properties and evolutionary stages of a sample of 17 clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared, millimeter, and radio data with new Bolocam Galactic Plane Survey (BGPS) 1.1 mm data, Very Large Array radio continuum data, and Heinrich Hertz Telescope dense gas (HCO⁺ and N₂H⁺) spectroscopic data. We combine literature studies of star formation tracers and dust temperatures within IRDCs with our search for ultracompact (UC) HII regions to discuss a possible evolutionary sequence for IRDC clumps. In addition, we perform an analysis of mass tracers in IRDCs and find that 8μm extinction masses and 1.1 mm BGPS masses are complementary mass tracers in IRDCs except for the most active clumps (notably those containing UC Hii regions), for which both mass tracers suffer biases. We find that the measured virial masses in IRDC clumps are uniformly higher than the measured dust continuum masses on the scale of ∼1 pc. We use ¹³CO, HCO⁺, and N₂H+ to study the molecular gas properties of IRDCs and do not see any evidence of chemical differentiation between hot and cold clumps on the scale of ∼1 pc. However, both HCO⁺ and N₂H⁺ are brighter in active clumps, due to an increase in temperature and/or density. We report the identification of four UC HII regions embedded within IRDC clumps and find that UC Hii regions are associated with bright (≳1 Jy) 24μm point sources, and that the brightest UC HII regions are associated with "diffuse red clumps" (an extended enhancement at 8μm). The broad stages of the discussed evolutionary sequence (from a quiescent clump to an embedded HII region) are supported by literature dust temperature estimates; however, no sequential nature can be inferred between the individual star formation tracers.