2012ApJ...756...60S

We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3 mm using the 22 m ATNF Mopra Telescope located in Australia. After determining kinematic distances, we eliminated clumps that are not located in IRDCs and clumps with a separation between them of less than one Mopra beam. Our final sample consists of 92 IRDC clumps. The most commonly detected molecular lines are (detection rates higher than 8%) N₂H⁺, HNC, HN¹³C, HCO⁺, H¹³CO⁺, HCN, C₂H, HC₃N, HNCO, and SiO. We investigate the behavior of the different molecular tracers and look for chemical variations as a function of an evolutionary sequence based on Spitzer IRAC and MIPS emission. We find that the molecular tracers behave differently through the evolutionary sequence and some of them can be used to yield useful relative age information. The presence of HNC and N₂H⁺ lines does not depend on the star formation activity. On the other hand, HC₃N, HNCO, and SiO are predominantly detected in later stages of evolution. Optical depth calculations show that in IRDC clumps the N₂H⁺ line is optically thin, the C₂ H line is moderately optically thick, and HNC and HCO⁺ are optically thick. The HCN hyperfine transitions are blended, and, in addition, show self-absorbed line profiles and extended wing emission. These factors combined prevent the use of HCN hyperfine transitions for the calculation of physical parameters. Total column densities of the different molecules, except C₂H, increase with the evolutionary stage of the clumps. Molecular abundances increase with the evolutionary stage for N₂H⁺ and HCO⁺. The N₂H⁺/HCO⁺ and N₂H⁺/HNC abundance ratios act as chemical clocks, increasing with the evolution of the clumps.