Molecular Line Observations of Infrared Dark Clouds: Seeking the Precursors to Intermediate and Massive Star Formation

We have identified 41 infrared dark clouds from the 8 μm maps of the Midcourse Space Experiment (MSX), selected to be found within 1 deg² areas centered on known ultracompact H II regions. We have mapped these infrared dark clouds in N₂H⁺1-->0, CS 2-->1, and C¹⁸O 1-->0 emission using the Five College Radio Astronomy Observatory. The maps of the different species often show striking differences in morphologies, indicating differences in evolutionary state and/or the presence of undetected, deeply embedded protostars. We derive an average mass for these clouds using N₂H⁺ column densities of ~2500 M_solar, a value comparable to that found in previous studies of high-mass star-forming cores using other mass tracers. The line widths of these clouds are typically ~2.0-2.9 km s^-1. Based on the fact that they are dark at 8 μm, compact, and massive, and have large velocity dispersions, we suggest that these clouds may be the precursor sites of intermediate- and high-mass star formation.