2017A&A...598A..76Z

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 multiwavelength observations on a small sample.
Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m telescopes, with tracers HCO⁺, HCN, N₂H⁺, C¹⁸O, 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 70 to 500 µm, we obtained dust temperature and column density distributions of the IRDCs. We found that N₂H⁺ emission has a strong correlation with the dust temperature and column density distributions, while C¹⁸O showed the weakest correlation. It is suggested that N₂H⁺ is indeed a good tracer in very dense conditions, but C¹⁸O 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⁺ and C¹⁸O toward the same core. This case can be well explained by model "envelope expansion with core collapse (EECC)".