2018RAA....18....4L

In this work, we perform a statistical investigation towards 50 high-mass clumps using data from the Bolocam Galactic Plane Survey (BGPS) and Millimetre Astronomy Legacy Team 90-GHz survey (MALT90). Eleven dense molecular lines (N₂H⁺ (1-0), HNC(1-0), HCO⁺ (1-0), HCN(1-0), HN¹³C(1-0), H¹³CO⁺ (1-0), C_ 2_H(1-0), HC₃ N(10-9), SiO(2-1),¹³CS(2-1)and HNCO(4_4,0 - 3_0,3)) are detected. N₂H ⁺ and HNC are shown to be good tracers for clumps in various evolutionary stages since they are detected in all the fields. The detection rates of N-bearing molecules decrease as the clumps evolve, but those of O-bearing species increase with evolution. Furthermore, the abundance ratios [N₂H⁺]/[HCO⁺] and log([HC₃N]/[HCO⁺]) decline with log([HCO⁺]) as two linear functions, respectively. This suggests that N₂H⁺ and HC₃N transform to HCO⁺ as the clumps evolve. We also find that C₂H is the most abundant molecule with an order of magnitude 10^–8. In addition, three new infall candidates, G010.214-00.324, G011.121-00.128 and G012.215-00.118(a), are discovered to have large-scale infall motions and infall rates with an order of magnitude 10^–3M_☉ yr^–1.