We have studied 14 southern infrared dark clouds ( IRDCs ) using the data taken from the Millimetre Astronomy Legacy Team 90 GHz ( MALT90 ) survey and the GLIMPSE and MIPSGAL mid-infrared survey of the inner Galaxy .
The physical and chemical characteristics of the 14 IRDCs are investigated using N _ { 2 } H ^ { + } ( 1-0 ) , HNC ( 1-0 ) , HCO ^ { + } ( 1-0 ) , and HCN ( 1-0 ) molecular lines .
We find that the 14 IRDCs are in different evolutionary stages from the ” starless ” to the sources with an UCHII region .
Three IRDCs are detected to have the star forming activities .
The integrated intensity ratios { \it I { } _ { HCO ^ { + } / HCN } \ / } , { \it I { } _ { N _ { 2 } H ^ { + } / HCN } \ / } , and { \it I { } _ { HNC / HCN } \ / } are all about 1.5 , which is different from the 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 estimate the abundances of the four molecules from 10 ^ { -11 } to 10 ^ { -9 } , and the average abundance ratios N _ { HNC } / N _ { HCN } = 1.47 \pm 0.50 , N _ { HNC } / N _ { HCO ^ { + } } = 1.74 \pm 0.22 , N _ { HCN } / N _ { HCO ^ { + } } = 1.21 \pm 0.41 .