Abstract
Massive star formation, unlike its lower mass sibling, is still not understood in terms of its environment, formation mechanism and evolutionary sequence. Infrared dark clouds (IRDCs), dense and dark silhouettes against bright Galactic background, are thought to be the cradles of massive star and cluster formation, being cold, dense, and massive enough to harbour massive protostars at some point of their life. However, due to distant and opaque nature of these objects, high-angular resolution radio and infrared observations are required to resolve the IRDC cores, where the star formation takes place. Previous studies show great diversity of IRDCs, ranging from dense quiescent clouds on the verge of star formation to relatively evolved regions with embedded massive protostars driving outflows and forming HII regions. Various tracers of IRDC evolution have been proposed, aiming to establish a coherent picture of IRDC diversity and evolutionary sequence. A presence of an embedded protostar heating up its environment could be inferred from measuring temperature of the IRDC core, with luminosity tracing its evolutionary stage. We present an analysis of evolutionary tracers in nine relatively quiescent infrared dark clouds, using both single-dish observations and available infrared data. We find decreasing trend in deuterium fractionation of N2H+ vs. gas temperature derived from ammonia, dust temperature, N2H+ line width and bolometric luminosity. Our findings suggest a general behavior of earliest massive star precursors and suggest the use of deuterium fractionation of N2H+ as a chemical clock in massive star-forming regions.