A Systemic Study of 14 Southern Infrared Dark Clouds with the N2H+, HNC,
  HCO+, and HCN Lines
  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 N2H+(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 I(HCO+)/I(HCN), I(N2H+)/I(HCN), and I(HNC)/I(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) = 1.4+
-0.50, N(HNC)/N(HCO+) = 1.74 + - 0.22, N(HCN)/N(HCO+)= 1.21 + - 0.41.
