A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds⋆,⋆⋆
Department of PhysicsUniversity of Helsinki, PO Box 64, 00014 Helsinki, Finland
Received: 3 September 2013
Accepted: 14 November 2013
Context. Infrared dark clouds (IRDCs) provide a useful testbed in which to investigate the genuine initial conditions and early stages of massive-star formation.
Aims. We attempt to characterise the chemical properties of a sample of 35 massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters.
Methods. The clumps are studied using the MALT90 (Millimetre Astronomy Legacy Team 90 GHz) line survey data obtained with the Mopra 22 m telescope. The survey covers 16 different transitions near 90 GHz. The spectral-line data are used in concert with our previous LABOCA (Large APEX BOlometer CAmera) 870 μm dust emission data.
Results. Eleven MALT90 transitions are detected towards the clumps at least at the 3σ level. Most of the detected species (SiO, C2H, HNCO, HCN, HCO+, HNC, HC3N, and N2H+) show spatially extended emission towards many of the sources. Most of the fractional abundances of the molecules with respect to H2 are 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 this trend is also seen for HNC and HC3N. An opposite trend is seen for the C2H and N2H+ abundances. 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 N2H+ abundance. The HNC/HCN and N2H+/HNC abundance ratios are derived to be near unity on average, while that of HC3N/HCN is ~10%. The N2H+/HNC ratio appears to increase as the clump evolves, while the HNC/HCO+ ratio shows the opposite behaviour.
Conclusions. The detected SiO emission is probably caused by shocks driven by outflows in most cases, although shocks resulting from the cloud formation process could also play a role. Shock-origin for the HNCO, HC3N, and CH3CN emission is also plausible. The average HNC/HCN ratio is in good agreement with those seen in other IRDCs, but gas temperature measurements would be neeeded to study its temperature dependence. Our results support the finding that C2H can trace the cold gas, and not just the photodissociation regions. The HC3N/HCN ratio appears to be comparable to the values seen in other types of objects, such as T Tauri disks and comets.
Key words: astrochemistry / stars: formation / ISM: abundances / ISM: clouds / ISM: molecules / radio lines: ISM
This publication is partly based on data acquired with the Atacama Pathfinder EXperiment (APEX) under programme 087.F-9315(A). APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.
Appendices and Table 3 are available in electronic form at http://www.aanda.org
© ESO, 2014