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

Chuan-Peng Zhang, Jing-Hua Yuan, Guang-Xing Li, Jian-Jun Zhou, Jun-Jie Wang

Published 2016-11-27Version 1

Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet they play a crucial role in breeding newly-formed stars. Aims. In order to further understand the dynamics, chemistry, and evolution of IRDCs, we carried out multi-wavelength observations towards a small sample. Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m telescopes, with tracers ${\rm HCO^+}$, HCN, ${\rm N_2H^+}$, ${\rm C^{18}O}$, DCO$^+$, SiO, and DCN towards 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 7 newly-identified cores. Towards the dense cores, we detected 6 DCO$^+$, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution (SED) fits of the \textit{Herschel} 70 to 500 $\mu$m, we obtain dust temperature and column density distributions of the IRDCs. We find that ${\rm N_2H^+}$ emission has a strong correlation with the dust temperature and column density distributions, while the worst is ${\rm C^{18}O}$. It is suggested that ${\rm N_2H^+}$ is indeed a good tracer in very dense condition, however an unreliable one is ${\rm C^{18}O}$, which has a relatively low critical density and is vulnerable to freeze out onto the surface of cold dust grains. The dynamics within IRDCs is active with infall, outflow, and collapse, the spectra are abundant especially in deuterium species. Conclusions. We observe many blueshifted and redshifted profiles, respectively, with ${\rm HCO^+}$ and ${\rm C^{18}O}$ towards the same core. This case can be well explained by model "envelope expansion with core collapse (EECC)".