2007ApJ...668..348B

Infrared dark clouds are expected to harbor sources in different, very young evolutionary stages. To better characterize these differences, we observed a sample of 43 massive infrared dark clouds, originally selected as candidate high-mass starless cores, with the IRAM 30 m telescope covering spectral line tracers of low-density gas, high-density gas, molecular outflows/jets and temperature effects. The SiO(2-1) observations reveal detections toward 18 sources. Assuming that SiO is exclusively produced by sputtering from dust grains, this implies that at least in 40% of this sample star formation is ongoing. A broad range of SiO line widths is observed (between 2.2 and 65 km/s), and we discuss potential origins for this velocity spread. While the low-density tracers ¹²CO(2-1) and ¹³CO(1-0) are detected in several velocity components, the high-density tracer H¹³CO⁺(1-0) generally shows only a single velocity component and is hence well suited for kinematic distance estimates of IRDCs. Furthermore, the H¹³CO⁺ line width is on average 1.5 times larger than that of previously observed NH₃(1, 1). This is indicative of more motion at the denser core centers, due to either turbulence or beginning star formation activity. In addition, we detect CH₃ CN toward only six sources, whereas CH₃ OH is observed toward approximately 40% of the sample. Estimates of the CH₃ CN and CH₃OH abundances are low with average values of 1.2x10^–10 and 4.3x10^–10, respectively. These results are consistent with chemical models at the earliest evolutionary stages of high-mass star formation. Furthermore, the CH₃ OH abundances compare well to recently reported values for low-mass starless cores.