A systemic study of 14 southern infrared dark clouds with N2H+, HNC, HCO+ and HCN lines

We studied 14 southern infrared dark clouds (IRDCs) using data taken from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey and the GLIMPSE and MIPSGAL mid-infrared surveys of the inner Galaxy. The physical and chemical characteristics of the 14 IRDCs were investigated using N2H+(1-0), HNC(1-0), HCO+(1-0) and HCN(1-0) molecular lines. We found that the 14 IRDCs are in different evolutionary stages, from 'starless' to sources with an ultra-compact H II region. Three IRDCs were detected to have star-forming activity. The integrated intensity ratios IHCO+/HCN, IN2H+/HCN and I-HNC/HCN are all similar to 1.5, which is different from previous measurements, suggesting that the integrated intensity ratios may be affected by the cloud environments. The integrated intensities of HNC, HCO+ and HCN show a tight correlation for the 14 IRDCs, implying a close link to the chemical evolution of these three species in the IRDCs. The derived excitation temperature for each IRDC is less than 25 K. We estimated the abundances of the four molecules to be from 10(-11) to 10(-9), and the average abundance ratios are N-HNC/N-HCN = 1.47 +/- 0.50, N-HNC/NHCO+ = 1.74 +/- 0.22 and N-HCN/NHCO+ = 1.21 +/- 0.41.