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We examine the physical properties and evolutionary stages of a sample of 17clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared,millimeter, and radio data with new Bolocam Galactic Plane Survey (BGPS) 1.1 mmdata, VLA radio continuum data, and HHT dense gas (HCO+ and N2H+) spectroscopicdata. We combine literature studies of star formation tracers and dusttemperatures within IRDCs with our search for ultra-compact (UC) HII regions todiscuss a possible evolutionary sequence for IRDC clumps. In addition, weperform an analysis of mass tracers in IRDCs and find that 8 micron extinctionmasses and 1.1 mm Bolocam Galactic Plane Survey (BGPS) masses are complementarymass tracers in IRDCs except for the most active clumps (notably thosecontaining UCHII regions), for which both mass tracers suffer biases. We findthat the measured virial masses in IRDC clumps are uniformly higher than themeasured dust continuum masses on the scale of ~1 pc. We use 13CO, HCO+, andN2H+ to study the molecular gas properties of IRDCs and do not see any evidenceof 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 increasein temperature and/or density. We report the identification of four UCHIIregions embedded within IRDC clumps and find that UCHII regions are associatedwith bright (>1 Jy) 24 micron point sources, and that the brightest UCHIIregions are associated with "diffuse red clumps" (an extended enhancement at 8micron). The broad stages of the discussed evolutionary sequence (from aquiescent clump to an embedded HII region) are supported by literature dusttemperature estimates; however, no sequential nature can be inferred betweenthe individual star formation tracers.

An Infrared through Radio Study of the Properties and Evolution of IRDC Clumps