Outflow and Dense Gas Emission from Massive Infrared Dark Clouds

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-1), 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 CH3CN toward only six sources, whereas CH3OH is observed toward approximately 40% of the sample. Estimates of the CH3CN and CH3OH abundances are low with average values of 1.2 × 10-10 and 4.3 × 10-10, respectively. These results are consistent with chemical models at the earliest evolutionary stages of high-mass star formation. Furthermore, the CH3OH abundances compare well to recently reported values for low-mass starless cores.