A statistical study towards the high-mass BGPS clumps with the MALT90
  survey
  In this work, we perform a statistical investigation towards 50 high-mass
clumps using the data from the Bolocam Galactic Plane Survey (BGPS) and the
Millimetre Astronomy Legacy Team 90-GHz survey (MALT90). Eleven dense molecular
lines (N$_2$H$^+$(1-0), HNC(1-0), HCO$^+$(1-0), HCN(1-0), HN$^{13}$C(1-0),
H$^{13}$CO$^+$(1-0), C$_2$H(1-0), HC$_3$N(10-9), SiO(2-1), $^{13}$CS(2-1) and
HNCO$(4_{4,0}-3_{0,3}))$ are detected. N$_2$H$^+$ and HNC are shown to be good
tracers for clumps in virous evolutionary stages since they are detected in all
the fields. And 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 [N$_2$H$^+$]/[HCO$^+$] and
Log([HC$_3$N]/[HCO$^+$]) decline with Log([HCO$^+$]) as two linear functions,
respectively. This suggests the transformation of N$_2$H$^+$ and HC$_3$N to
HCO$^+$ as the clumps evolve. We also find that C$_2$H is the most abundant
molecule with an order of $10^{-8}$. Besides, three new infall candidates
G010.214-00.324, G011.121-00.128, and G012.215-00.118(a) are discovered to have
large-scaled infall motions and infall rates in the magnitude of $10^{-3}$
M$_\odot$ yr$^{-1}$.
