Chemical composition and chemical processes in IRDCs Tatiana Vasyunina University of Virginia, Charlottesville, USA
Is there anything special regarding the chemistry of the earliest phases of massive star formation? Should we build new chemical models to reproduce molecular line observations in infrared dark clouds (IRDCs) and high-mass protostellar objects embedded therein, or can we just use already existing recipes for low-mass starless cores and protostellar clouds? Our present study shows the importance of building new models for massive stars taking into account their specific physical conditions. For this investigations we selected a new sample of 15 IRDCs in the southern hemisphere. Using the radio telescopes Mopra, Parkes and APEX, we collected molecular line information in the frequency ranges 77-115 GHz, and 214-244 GHz. There we had detections for 17 different species including CO and its isotopologues, tracers of cold and dense material (HCCCN, HNC, HCN, etc.), nitrogen- (NH3, N2H+) and sulfur-bearing (SO, CS) species as well as more complex organic molecules like methanol. Available 1.2 mm SIMBA/SEST data (Vasyunina et al. 2009) allowed us not only calculate column densities for detected species, but also estimate molecular abundances. The next step was to perform chemical modeling using available molecular species for model justification. We developed chemical models with a simplified internal structure and tested it for two representative IRDCs: IRDC013.90-1 and IRDC321.73-1. Both clouds are relatively well studied (Vasyunina et al. 2009, 2011) and may represent two distinct temperature regimes. IRDC013.90-1 is a colder cloud with T = 15 K, whereas IRDC321.73-1 is somewhat warmer with T = 25 K, based on our NH3 measurements. We compared our results with observed molecular abundances for eight species: N$_2$H$^+$, HC$_3$N, HNC, HCO$^+$, HCN, C$_2$H, NH$_3$ and CS, and distinguished a limited temperature range of 20 - 30 K, where grain-surface chemistry becomes important for gas-phase abundances of certain species such as CO, HCO$^+$, and N$_2$H$^+$. Hence, those IRDCs that have temperatures in the 20 - 30 K range can be an interesting laboratory to study the impact of surface reactions on the abundances of simple gas-phase species. That makes the chemistry in "warmer" IRDCs essentially different from the similar models for low-mass stars, where temperatures are typically lower.
Collaborators: A. Vasyunin, UVa, USA E. Herbst, UVa, USA H. Linz, MPIA, Germany M. Voronkov, CSIRO, Australia I. Zinchenko, APPL, Russia