2010ApJ...714.1658S

We have observed the CH₃ OH J = 2-1, SiO J = 2-1, C³⁴S J = 2-1, H¹³CO⁺ J = 1-0, HN¹³C J = 1-0, CCH N = 1-0, OCS J = 8-7, and SO J_N= 2₂-1₁ lines toward 20 massive clumps, including Midcourse Space Experiment (MSX) 8 µm dark sources (infrared dark clouds) and MSX 8 µm sources, by using the Nobeyama Radio Observatory 45 m telescope. We have found that the velocity widths of the CH₃ OH and C³⁴S lines are broader than those of the H¹³CO⁺ line in the MSX dark sources. On the other hand, they are comparable to the velocity width of the H¹³CO⁺ line in the MSX sources. In addition, the [SiO]/[H¹³CO⁺] abundance ratio is found to be enhanced in the MSX dark sources in comparison with the MSX sources. These results suggest that shocks caused by interaction between an outflow and an ambient dense gas would have substantial impact on the chemical composition of the MSX dark sources. The velocity widths of the CH₃ OH and C³⁴S lines relative to that of the H¹³CO⁺ line as well as the [SiO]/[H¹³CO⁺] abundance ratio could be used as good tools for investigating evolutionary stages of massive clumps. On the basis of the results, we discuss the chemical and physical evolution of massive clumps.