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A multi-wavelength observation and investigation of six infrared dark clouds

Abstract

Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet they play a crucial role in breeding newly-formed stars. Aims. With the aim of further understanding the dynamics, chemistry, and evolution of IRDCs, we carried out multi-wavelength observations on a small sample. Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m telescopes, with tracers HCO+{\rm HCO^+}, HCN, N2H+{\rm N_2H^+}, C18O{\rm C^{18}O}, DCO+^+, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01, G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05. Results. We investigated 44 cores including 37 cores reported in previous work and seven newly-identified cores. Toward the dense cores, we detected 6 DCO+^+, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution (SED) fits of the Herschel\textit{Herschel} 70 to 500 μ\mum, we obtained dust temperature and column density distributions of the IRDCs. We found that N2H+{\rm N_2H^+} emission has a strong correlation with the dust temperature and column density distributions, while C18O{\rm C^{18}O} showed the weakest correlation. It is suggested that N2H+{\rm N_2H^+} is indeed a good tracer in very dense conditions, but C18O{\rm C^{18}O} is an unreliable one, as it has a relatively low critical density and is vulnerable to freezing-out onto the surface of cold dust grains. The dynamics within IRDCs are active, with infall, outflow, and collapse; the spectra are abundant especially in deuterium species. Conclusions. We observe many blueshifted and redshifted profiles, respectively, with HCO+{\rm HCO^+} and C18O{\rm C^{18}O} toward the same core. This case can be well explained by model "envelope expansion with core collapse (EECC)".Comment: 24 pages, 11 figures, 4 tables. To be published in A&A. The resolutions of the pictures are cut dow

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