2019MNRAS.489.4497Y

We investigated the chemical evolution of HC₃N in six dense molecular clouds, using archival available data from the Herschel infrared Galactic Plane Survey (Hi-GAL) and the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). Radio sky surveys of the Multi-Array Galactic Plane Imaging Survey (MAGPIS) and the Sydney University Molonglo Sky Survey (SUMSS) indicate these dense molecular clouds are associated with ultracompact H II (UCH II) regions and/or classical H II regions. We find that in dense molecular clouds associated with normal classical H II regions, the abundance of HC₃N begins to decrease or reaches a plateau when the dust temperature gets hot. This implies UV photons could destroy the molecule of HC₃N. On the other hand, in the other dense molecular clouds associated with UCH II regions, we find the abundance of HC₃N increases with dust temperature monotonously, implying HC₃N prefers to be formed in warm gas. We also find that the spectra of HC₃N (10-9) in G12.804-0.199 and RCW 97 show wing emissions, and the abundance of HC₃N in these two regions increases with its non-thermal velocity width, indicating HC₃N might be a shock origin species. We further investigated the evolutionary trend of N(N₂H⁺)/N(HC₃N) column density ratio, and found this ratio could be used as a chemical evolutionary indicator of cloud evolution after the massive star formation is started.