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A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

We attempt to characterise the chemical properties of a sample of massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters. The clumps are studied using the MALT90 line survey data obtained with the Mopra telescope. The spectral-line data are used in concert with our previous LABOCA 870-$μ$m dust emission data. Most of the detected species (SiO, C$_2$H, HNCO, HCN, HCO$^+$, HNC, HC$_3$N, and N$_2$H$^+$) show spatially extended emission towards many of the sources. The fractional abundances of the molecules are mostly found to be comparable to those determined in other recent similar studies of IRDC clumps. We found that the abundances of SiO, HNCO, and HCO$^+$ are higher in IR-bright clumps than in IR-dark sources, reflecting a possible evolutionary trend. A hint of such a trend is also seen for HNC and HC$_3$N. An opposite trend is seen for C$_2$H and N$_2$H$^+$. Moreover, a positive correlation is found between the abundances of HCO$^+$ and HNC, and between those of HNC and HCN. The HCN and HNC abundances also appear to increase as a function of the N$_2$H$^+$ abundance. The HNC/HCN and N$_2$H$^+$/HNC abundance ratios are derived to be ~1 on average, while that of HC$_3$N/HCN is ~10%. The N$_2$H$^+$/HNC ratio appears to increase as the clump evolves, while the HNC/HCO$^+$ ratio shows the opposite behaviour. The detected SiO emission is likely caused by outflow shocks in most cases, although shocks resulting from the cloud formation process could also play a role. Shock-origin for the HNCO, HC$_3$N, and CH$_3$CN emission is also plausible. The average HNC/HCN ratio is in good agreement with those seen in other IRDCs. Our results support the finding that C$_2$H can trace the cold gas, and not just the PDRs. The HC$_3$N/HCN ratio appears to be comparable to that in T Tauri disks and comets.