A Molecular Line Observation toward Massive Clumps Associated with Infrared Dark Clouds

We have surveyed the N₂H⁺ J = 1–0, HC₃N J = 5–4, CCS J_N = 4₃–3₂, NH₃ (J, K) = (1, 1), (2, 2), (3, 3), and CH₃OH J = 7–6 lines toward the 55 massive clumps associated with infrared dark clouds by using the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experiment 10 m telescope. The N₂H⁺, HC₃N, and NH₃ lines are detected toward most of the objects. On the other hand, the CCS emission is detected toward none of the objects. The [CCS]/[N₂H⁺] ratios are found to be mostly lower than unity even in the Spitzer 24 μm dark objects. This suggests that most of the massive clumps are chemically more evolved than the low-mass starless cores. The CH₃OH emission is detected toward 18 out of 55 objects. All the CH₃OH-detected objects are associated with the Spitzer 24 μm sources, suggesting that star formation has already started in all the CH₃OH-detected objects. The velocity widths of the CH₃OH J_K = 7₀–6₀ A⁺ and 7₋₁–6₋₁ E lines are broader than those of N₂H⁺ J = 1–0. The CH₃OH J_K = 7₀–6₀ A⁺ and 7₋₁–6₋₁ E lines tend to have broader line width in the MSX dark objects than in the others, the former being younger or less luminous than the latter. The origin of the broad emission is discussed in terms of the interaction between an outflow and an ambient cloud.