Knowledge Bank

Here we present our molecular line investigation of the southern infrared dark clouds (IRDCs). We performed observations of 13 molecular species using the 22-m Mopra radio telescope. In our survey we included in general species which are good tracers of cold and dense gas. Some of them trace the more quiescent gas (e.g. HNC, N$2$H$+$), while others are sensitive to more dynamical processes (HCN, HCO$+$). We detect SiO emission in some clouds and complicated shapes of the HCO$+$ emission line profile in all IRDCs, which indicates infall and outflow motions and the beginning of starformation activity, at least in some parts of the IRDCs. Using H$2$ column densities from our previous investigation, we estimated molecular abundances for all species. We uncovered a tendency for IRDCs to have molecular abundances similar to those in low-mass pre-stellar cores. This similarity may indicate similar chemical composition at the earliest stages of low- and high-mass stars and their close evolutionary status. However, for the N$2$H$+$ and HCO$+$ species, there is a tendency for higher mean abundances in IRDCs compared with low- mass cores. To find a reason of such behavior, we use chemical models including gas-phase reactions and accretion and desorption onto/from grains. Chemical modeling allows us to study chemical evolution at the early stages of massive stars in more detail and reconstruct the physical conditions and evolutionary status of IRDCs.